Structure having balanced pH profile

ABSTRACT

Disclosed is an absorbent structure comprising an acidic or basic water-swellable, water-insoluble polymer, a basic or acidic second material, and, optionally, a buffering agent, wherein the absorbent structure exhibits desirable absorbent properties. Specifically, the present invention relates to an absorbent structure having the ability to absorb a large quantity of liquid while maintaining a substantially desired and balanced pH profile on or along the upper surface of the absorbent structure. The absorbent structure is useful in disposable absorbent products, such as those disposable absorbent products that are used to absorb body liquid.

This application is a divisional continuation application Ser. No.10/627,061 and filed on Jul. 25, 2003, now U.S. Pat. No. 7,012,105 whichis a continuation of U.S. application Ser. No. 09/188,358, filed on Nov.10, 1998 and now U.S. Pat. No. 6,639,120, which is acontinuation-in-part of U.S. application Ser. No. 08/989,555, filed onDec. 12, 1997 and now abandoned, the disclosures of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an absorbent structure comprising anacidic or basic water-swellable, water-insoluble polymer, a basic oracidic material, and, optionally, a buffering agent, wherein theabsorbent structure exhibits desirable absorbent properties.Specifically, the present invention relates to an absorbent structurehaving the ability to absorb a large quantity of liquid whilemaintaining a substantially desired and balanced pH profile on or alongthe upper surface of the absorbent structure. The absorbent structure isuseful in disposable absorbent products, such as those disposableabsorbent products that are used to absorb bodily liquids.

2. Description of the Related Art

The use of water-swellable, generally water-insoluble absorbentmaterials, commonly known as superabsorbents, in disposable absorbentproducts is known. Such absorbent materials are generally employed indisposable absorbent products such as diapers, training pants, adultincontinence products, and feminine care products in order to increasethe absorbent capacity of such products while reducing their overallbulk. Such absorbent materials are generally present in disposableabsorbent products in a fibrous matrix, such as a matrix of wood pulpfluff. A matrix of wood pulp fluff generally has an absorbent capacityof about 6 grams of liquid per gram of fluff. The superabsorbentmaterials generally have an absorbent capacity of at least about 10,preferably of about 20, and often of up to 100 times their weight inwater. Clearly, incorporation of such absorbent materials in disposableabsorbent products can reduce the overall bulk while increasing theabsorbent capacity of such products.

The superabsorbent material commonly used in disposable absorbentproducts is the substantially neutralized form of a crosslinked polymer,such as the sodium salt of a crosslinked polyacrylic acid. The salt formof a crosslinked polymer is generally used since the capacity for theabsorption of aqueous liquids of a crosslinked but substantiallyunneutralized polymer is typically very low as compared to theneutralized, or salt, form of the crosslinked polymer. However, onepotential beneficial aspect of using the unneutralized form of acrosslinked polymer is that such a material has the capacity to exchangesome of the cations present in urine and other body liquids thattypically insult disposable absorbent products. In contrast, thesubstantially neutralized form of a crosslinked polymer generally doesnot allow for such ion-exchanging to occur.

It is, therefore, an object of the present invention to use thesubstantially unneutralized form of a crosslinked polymer in adisposable absorbent product in combination with another material whichwould neutralize the crosslinked polymer in situ when urine or otherbody liquids contact the disposable absorbent product. The use of thesubstantially unneutralized form of a crosslinked polymer would helpreduce the ionic content of the body liquid through ion-exchange. Thereduction in the ionic strength of a body liquid contacting a disposableabsorbent product is generally beneficial in that the absorbent capacityof a crosslinked polymer is generally inversely related to the ionicstrength of the liquid being absorbed. Furthermore, the synthesis of thesubstantially unneutralized form of a crosslinked polymer generallyprovides a better crosslinked polymeric network as compared to thesynthesis of the substantially neutralized form of the crosslinkedpolymer, in that the formation of polymeric network defects is generallyminimized which tends to increase the absorbent capacity of thecrosslinked polymer. Therefore, another potential benefit of using thesubstantially unneutralized form of a crosslinked polymer, which isneutralized in situ, in a disposable absorbent product is theimprovement in liquid absorption and distribution that occurs in thedisposable absorbent product as problems caused by the rapid swelling ofthe substantially neutralized form of the crosslinked polymer can beavoided.

A complexity to the use of a substantially unneutralized form of acrosslinked polymer, with in situ neutralization of the crosslinkedpolymer, is the need to maintain a balanced pH profile on the surface ofthe disposable absorbent product. As the crosslinked polymer is beingneutralized in situ after contact with a body liquid, temporaryimbalances in pH can occur due to differences in the rate of dissolutionor ionization of the material being used to neutralize the crosslinkedpolymer and the diffusion of the ionic species to the unneutralizedsites in the crosslinked polymer which allows neutralization to occur.This temporary pH imbalance could result in an undesirable alkaline pHor an acidic pH in the disposable absorbent product next to a user'sskin with the potential for skin irritation. As such, there is a needfor controlling the pH in that portion of the disposable absorbentproduct that comes into contact with or is otherwise near or next to awearer's or user's skin. By better controlling the pH in that portion ofthe disposable absorbent product that comes into contact with or isotherwise near or next to a user's skin, the incidence of skinirritation should be reduced.

It is, therefore, an object of the present invention to provide anabsorbent structure that absorbs a large quantity of liquid, such as atabout the same final capacity as compared to an absorbent structurecomprising commercially available superabsorbent materials, wherein theabsorbent structure maintains a substantially desired and balanced pHprofile on or along the upper surface of the absorbent structure.

It is also an object of the present invention to provide an absorbentstructure that comprises a substantially unneutralized form of acrosslinked polymer in combination with an inexpensive material thatneutralizes the crosslinked polymer in situ since such an approach mayreduce the overall cost of the absorbent structure.

It is also an object of the present invention to provide an absorbentstructure that may be prepared simply and with a minimum of materialsand additives so as to reduce the overall cost of preparing theabsorbent structure as well as reduce the potential deleterious effectthat such additives might have on the overall absorbent properties ofthe absorbent structure.

It is also an object of the present invention to provide an absorbentstructure that exhibits unique properties so that such absorbentstructure may be used in novel applications.

SUMMARY OF THE INVENTION

In one aspect, the present invention concerns an absorbent structurehaving the ability to absorb a large quantity of liquid whilemaintaining a substantially desired and balanced pH profile along theupper surface of the absorbent structure.

One embodiment of the present invention concerns an absorbent structureto be used in contact with human skin of a wearer, the absorbentstructure comprising an upper surface oriented toward the skin of thewearer and a lower surface oriented away from the skin of the wearer,the absorbent structure further comprising:

-   -   a) a water-swellable, water-insoluble polymer having acidic        functional groups, wherein the water-swellable, water-insoluble        polymer has at least about 50 molar percent of the acidic        functional groups in free acid form; and    -   b) a basic material;        wherein the absorbent structure exhibits a Wicking Capacity        value that is at least about 5 grams per gram of absorbent        structure and exhibits a pH on the upper surface that remains        within the range of about 3 to about 8.

Another embodiment of the present invention concerns an absorbentstructure to be used in contact with human skin of a wearer, theabsorbent structure comprising an upper surface oriented toward the skinof the wearer and a lower surface oriented away from the skin of thewearer, the absorbent structure further comprising:

-   -   a) a water-swellable, water-insoluble polymer having basic        functional groups, wherein the water-swellable, water-insoluble        polymer has at least about 50 molar percent of the basic        functional groups in free acid form; and    -   b) an acidic material;        wherein the absorbent structure exhibits a Wicking Capacity        value that is at least about 5 grams per gram of absorbent        structure and exhibits a pH on the upper surface that remains        within the range of about 3 to about 8.

In another aspect, the present invention concerns a disposable absorbentproduct comprising an absorbent structure of the present invention thatexhibits desired absorbent and pH properties.

In one embodiment of the present invention, a disposable absorbentproduct comprises a liquid-permeable topsheet, a backsheet attached tothe topsheet, and an absorbent structure positioned between theliquid-permeable topsheet and the backsheet wherein the absorbentstructure exhibits desired liquid absorbent and pH-control properties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of the equipment employed in determining theFree Swell and Absorbency Under Load values of an absorbent composition.

FIG. 2 is an illustration of the equipment employed in determining theWicking Capacity of an absorbent structure as well as the pH valuesalong the upper surface of an absorbent structure.

FIG. 3 is an illustration of the equipment employed in determining theIonization Rate of a material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has now been discovered that an absorbent structure may be preparedthat exhibits a relatively high total liquid absorption capacity as wellas maintaining a substantially desired and balanced pH profile on oralong the upper surface of the absorbent structure. In accordance withthis invention, the absorbent structure comprises an upper surfaceoriented toward the skin of a wearer and a lower surface oriented awayfrom the skin of the wearer. As used herein, “upper surface” means thatsurface of an absorbent structure which is intended to be worn toward oradjacent to the body of a wearer, while the “lower surface” is on thegenerally opposite side of the absorbent structure from the uppersurface and is intended to be worn away from the wearer's body buttowards, for example, any undergarments when the absorbent structure isworn.

The absorbent structure of the present invention generally comprises atleast two different components. The first component is awater-swellable, water-insoluble polymer. As used in the absorbentstructure of the present invention, the water-swellable, water-insolublepolymer to a large extent needs to provide the absorbent structure withits liquid-absorbing capacity. As such, the water-swellable,water-insoluble polymer needs to be effective to provide a desiredamount of liquid-absorbing capacity to the absorbent structure.

As used herein, the terms “comprise”, “comprises”, “comprising”, orsimilar terms, are intended to be synonymous with “including”, “having”,“containing”, or “characterized by”, and are intended to be inclusive oropen-ended and are not intended to exclude additional, unrecitedcomponents, elements, or method steps.

As used herein, the term “water-swellable, water-insoluble” is meant torefer to a material that, when exposed to an excess of water, swells toits equilibrium volume but does not dissolve into the solution. As such,a water-swellable, water-insoluble material generally retains itsoriginal identity or physical structure, but in a highly expanded state,during the absorption of the water and, thus, must have sufficientphysical integrity to resist flow and fusion with neighboring particles.

As used herein, a material will be considered to be “water soluble” whenit substantially dissolves in excess water to form a solution, therebylosing its initial, typically particulate, form and becoming essentiallymolecularly dispersed throughout the water solution. As a general rule,a water-soluble material will be free from a substantial degree ofcrosslinking, as crosslinking tends to render a material waterinsoluble.

One property of the water-swellable, water-insoluble polymer which isrelevant to its effectiveness in providing a desired amount ofliquid-absorbing capacity to the absorbent structure is its molecularweight. In general, a water-swellable, water-insoluble polymer with ahigher molecular weight will exhibit a higher liquid-absorbing capacityas compared to a water-swellable, water-insoluble polymer with a lowermolecular weight.

The water-swellable, water-insoluble polymer useful in the absorbentstructure may generally have a wide range of molecular weights. Awater-swellable, water-insoluble polymer having a relatively highmolecular weight is often beneficial for use in the present invention.Nonetheless, a wide range of molecular weights is generally suitable foruse in the present invention. Water-swellable, water-insoluble polymerssuitable for use in the present invention will beneficially have aweight average molecular weight greater than about 100,000, morebeneficially greater than about 200,000, suitably greater than about500,000, more suitably greater than about 1,000,000, and up to about10,000,000. Methods for determining the molecular weight of a polymerare known to those skilled in the art.

It is sometimes more convenient to express the molecular weight of apolymer in terms of its viscosity in a 1.0 weight percent aqueoussolution at 25° C. Polymers suitable for use in the present inventionwill suitably have a viscosity in a 1.0 weight percent aqueous solutionat 25° C. of from about 100 centipoise (100 mPa·s) to about 80,000centipoise (80,000 mPa·s), more suitably from about 500 centipoise (500mPa·s) to about 80,000 centipoise (80,000 mPa·s), and most suitably fromabout 1,000 centipoise (1,000 mPa·s) to about 80,000 centipoise (80,000mPa·s).

The water-swellable, water-insoluble polymer useful in the absorbentcomposition will generally be crosslinked. The amount of crosslinkingshould generally be above a minimum amount sufficient to make thepolymer water-insoluble but also below some maximum amount so as toallow the polymer to be sufficiently water swellable so that thewater-swellable, water-insoluble polymer absorbs a desired amount ofliquid.

Crosslinking of the polymer may generally be achieved by either of twodifferent types of crosslinking agents. The first type of crosslinkingagent is a polymerizable crosslinking agent. Suitable polymerizablecrosslinking agents are generally reactive to the monomer or monomersused to prepare the polymer and, thus, generally comprise at least twofunctional groups that are capable of reacting with the monomers.Examples of suitable polymerizable crosslinking agents includeethylenically unsaturated monomers, such as N,N′-methylenebis-acrylamide, for free radical polymerization and polyamines orpolyols for condensation polymerization.

The second type of crosslinking agent is a latent crosslinking agent.Latent crosslinking agents generally can be either polymerizable ornon-polymerizable. The non-polymerizable crosslinking agents generallydo not take part in the overall polymerization process but, instead, arereactive to the polymer at a later point in time when a propercrosslinking condition is provided. The polymerizable crosslinkingagents do take part in the overall polymerization process but generallydo not cause intermolecular crosslinking. The intermolecularcrosslinking generally only occurs at a later point in time when aproper crosslinking condition is provided. Suitable post treatmentconditions include using heat treatment, such as a temperature aboveabout 60° C., exposure to ultraviolet light, exposure to microwaves,steam or high humidity treatment, high pressure treatment, or treatmentwith an organic solvent.

Latent non-polymerizable crosslinking agents suitable for use in thepresent invention are generally water soluble. A suitable latentnon-polymerizable crosslinking agent is an organic compound having atleast two functional groups or functionalities capable of reacting withany carboxyl, carboxylic, amino, or hydroxyl groups on the polymer.Examples of suitable latent non-polymerizable crosslinking agentsinclude, but are not limited to, diamines, polyamines, diols, polyols,polycarboxylic acids, and polyoxides. Another suitable latentnon-polymerizable crosslinking agent comprises a metal ion with morethan two positive charges, such as Al³⁺, Fe³⁺, Ce³⁺, Ce⁴⁺, Ti⁴⁺, Zr⁴⁺,and Cr³⁺.

When the polymer is a cationic polymer, a suitable latentnon-polymerizable crosslinking agent is a polyanionic material such assodium polyacrylate, carboxymethyl cellulose, or polyphosphate.

Latent polymerizable crosslinking agents suitable for use in the presentinvention are generally water soluble and reactive to the monomer ormonomers used to prepare the water-swellable, water-insoluble polymer.The latent polymerizable crosslinking agents generally contain at leastone functional group or functionality capable of reacting with themonomer or monomers and at least one functional group or functionalitycapable of reacting with any carboxyl, carboxylic, amino, or hydroxylgroups on the polymer. Examples of suitable latent polymerizablecrosslinking agents include, but are not limited to, ethylene glycolvinyl ether, amino propanol vinyl ether, diethylamino ethylmethacrylate, allylamine, methylallylamine, ethylallylamine.

In one embodiment of the present invention, the water-swellable,water-insoluble polymer useful in the absorbent structure will be acidicin nature. As used herein, an “acidic” material is intended to refer toa material that may act as an electron acceptor and which, in an aqueoussolution, exhibits a pH between about 0 to 7. Suitably, the pH ismeasured at about 25° C. Methods of measuring the pH of an aqueoussolution are well known in the art.

In general, acidic, water-swellable, water-insoluble polymers useful inthe absorbent structure may be either strongly or weakly acidic innature. In general, an acidic, water-swellable, water-insoluble polymerthat is strongly acidic will exhibit a pKa less than about 2. Ingeneral, an acidic, water-swellable, water-insoluble polymer that isweakly acidic will exhibit a pKa that is greater than about 2. As such,acidic, water-swellable, water-insoluble polymers useful in theabsorbent structure may exhibit a broad range of pKa values, but willbeneficially have a pKa between about 0 to about 12, more beneficiallybetween about 2 to about 10, and suitably between about 3 to about 7. Aswill be appreciated by one skilled in the art, a monobasic acid willgenerally have a single pKa value whereas multibasic acids willgenerally have multiple pKa values. Unless indicated otherwise herein, areference to the pKa value of a multibasic acid is intended to refer tothe pKa₁ value of the multibasic acid.

It may sometimes be more convenient to measure the pKa of the monomer ormonomers used to prepare a polymer. Although the pKa of the monomer ormonomers and the polymer prepared from such monomers may not beidentical, such pKa values should be substantially similar. As such,acidic, water-swellable, water-insoluble polymers useful in theabsorbent structure may be prepared from a single monomer or acombination of monomers that exhibit a broad range of pKa values, butsuch monomers will beneficially have a pKa between about 0 to about 12,more beneficially between about 2 to about 10, and suitably betweenabout 3 to about 7.

The pKa of an acid represents the extent of dissociation of or, in otherwords, the strength of the acid and is intended herein to be measured atthe conditions, such as at a specific temperature, under which thewater-swellable, water-insoluble polymer is being used. Suitably, thepKa is measured at about 25° C. In general, the weaker the acid, thehigher the pKa value will be. The pKa values for many acids at varioustemperatures are well known and may be found in any of many availablereferences, such as in the CRC Handbook of Chemistry & Physics, 75^(th)Edition, edited by David R. Lide, CRC Press (1994).

Suitable acidic, water-swellable, water-insoluble polymers will includefunctional groups that are capable of acting as acids. Such functionalgroups include, but are not limited to, carboxyl groups, sulfonicgroups, sulphate groups, sulfite groups, and phosphate groups. Suitably,the functional groups are carboxyl groups. Generally, the functionalgroups are attached to a crosslinked base polymer. Suitable basepolymers include polyacrylamides, polyvinyl alcohols, ethylene maleicanhydride copolymer, polyvinylethers, polyacrylamido methylpropanesulfonic acid, polyacrylic acids, polyvinylpyrrolidones,polyvinylmorpholines, and copolymers of the preceding polymers. Naturalbased polysaccharide polymers may also be used and include carboxymethylcelluloses, carboxymethyl starches, hydroxypropyl celluloses, algins,alginates, carrageenans, acrylic grafted starches, acrylic graftedcelluloses, and copolymers of the preceding polymers. Syntheticpolypeptides can also be used such as polyaspartic acid and polyglutamicacid.

The acidic, water-swellable, water-insoluble polymer generally needs tobe in its free acid form. In general, it is desired that the acidic,water-swellable, water-insoluble polymer beneficially have at leastabout 50 molar percent, more beneficially at least about 70 molarpercent, suitably at least about 80 molar percent, more suitably atleast about 90 molar percent, and most suitably substantially about 100molar percent of its acidic functional groups in free acid form.Alternatively, then, the acidic, water-swellable, water-insolublepolymer should not be substantially neutralized when used in theabsorbent structure of the present invention. In general, it is desiredthat the acidic, water-swellable, water-insoluble polymer have a degreeof neutralization of its acidic functional groups that is beneficiallyless than about 50 molar percent, more beneficially less than about 30molar percent, suitably less than about 20 molar percent, more suitablyless than about 10 molar percent, and most suitably substantially about0 molar percent.

Commercially available superabsorbents are generally in a substantiallyneutralized or salt form. This is because, in general, in order to havea relatively high capacity for liquid absorption, a water-swellable,water-insoluble polymer must be a polyelectrolyte. However, as discussedherein, the acidic water-swellable, water-insoluble polymers useful inthe present invention are substantially in their free acid form.Therefore, such acidic water-swellable, water-insoluble polymers intheir free acid form generally do not have, on their own, a relativelyhigh capacity for liquid absorption.

It has been discovered in the present invention, however, that when suchan acidic water-swellable, water-insoluble polymer, substantially in itsfree acid form, is combined or mixed with a basic second material, theresulting combination or mixture will exhibit a relatively high capacityfor liquid absorption. This is believed to be because as the mixture isplaced in an aqueous solution, the acidic water-swellable,water-insoluble polymer, substantially in its free acid form, reactswith the basic second material, and the chemical equilibrium is in favorof converting the acidic water-swellable, water-insoluble polymer fromits free acid form to its respective salt form. As such, the mixturecomprising the substantially neutralized water-swellable,water-insoluble polymer will now exhibit a relatively high capacity forliquid absorption. In addition, the conversion of the water-swellable,water-insoluble polymer, from its free acid form to its respective saltform in an electrolyte-containing solution, such as an aqueous sodiumchloride solution or urine, can have a substantial desalting effect onthe electrolyte-containing solution, thereby improving theliquid-absorbing performance of the mixture comprising thewater-swellable, water-insoluble polymer by alleviating anysalt-poisoning effect.

In another embodiment of the present invention, the water-swellable,water-insoluble polymer useful in the absorbent structure will be basicin nature. As used herein, a “basic” material is intended to refer to amaterial that may act as an electron donor and which, in an aqueoussolution, exhibits a pH between 7 to about 14. Suitably, the pH ismeasured at about 25° C. Methods of measuring the pH of an aqueoussolution are well known in the art.

In general, basic, water-swellable, water-insoluble polymers useful inthe absorbent structure may be either strongly or weakly basic innature. In general, a basic, water-swellable, water-insoluble polymerthat is strongly basic will exhibit a pKa greater than about 12. Ingeneral, a basic, water-swellable, water-insoluble polymer that isweakly basic will exhibit a pKa that is less than about 12. As such,basic, water-swellable, water-insoluble polymers useful in the absorbentstructure may exhibit a broad range of pKa values, but will beneficiallyhave a pKa between about 2 to about 14, more beneficially between about4 to about 12, and suitably between about 7 to about 11. As will beappreciated by one skilled in the art, a monobasic base will generallyhave a single pKa value whereas multibasic bases will generally havemultiple pKa values. Unless indicated otherwise herein, a reference tothe pKa value of a multibasic base is intended to refer to the pKa₁value of the multibasic base.

It may sometimes be more convenient to measure the pKa of the monomer ormonomers used to prepare a polymer. Although the pKa of the monomer ormonomers and the polymer prepared from such monomers may not beidentical, such pKa values should be substantially similar. As such,basic, water-swellable, water-insoluble polymers useful in the absorbentstructure may be prepared from a single monomer or a combination ofmonomers that exhibit a broad range of pKa values, but such monomerswill beneficially have a pKa between about 2 to about 14, morebeneficially between about 4 to about 12, and suitably between about 7to about 11.

The pKa of an base represents the extent of dissociation of or, in otherwords, the strength of the base and is intended herein to be measured atthe conditions, such as at a specific temperature, under which thewater-swellable, water-insoluble polymer is being used. Suitably, thepKa is measured at about 25° C. In general, the weaker the base, thelower the pKa value will be. The pKa values for many bases at varioustemperatures are well known and may be found in any of many availablereferences, such as in the CRC Handbook of Chemistry & Physics, 75^(th)Edition, edited by David R. Lide, CRC Press (1994).

Suitable basic water-swellable, water-insoluble polymers will includefunctional groups that are capable of acting as bases. Such functionalgroups include, but are not limited to, primary, secondary, or tertiaryamino groups, imino groups, imido groups, amido groups, and quaternaryammonium groups. Suitably the functional groups are primary amino groupsor quaternary ammonium groups. Generally, the functional groups areattached to a crosslinked base polymer. Suitable base polymers includepolyamines, polyethyleneimines, polyacrylamides, polydiallyl dimethylammonium hydroxide, and polyquaternary ammoniums, and copolymersthereof. Natural based polysaccharide polymers may also be used andinclude chitin and chitosan. Synthetic polypeptides can also be usedsuch as polyasparagines, polyglutamines, polylysines, and polyarginines.

The basic, water-swellable, water-insoluble polymer generally needs tobe in its free base form. In general, it is desired that the basic,water-swellable, water-insoluble polymer beneficially have at leastabout 50 molar percent, more beneficially at least about 70 molarpercent, suitably at least about 80 molar percent, more suitably atleast about 90 molar percent, and most suitably substantially about 100molar percent of its basic functional groups in free base form.Alternatively, then, the basic, water-swellable, water-insoluble polymershould not be substantially neutralized when used in the absorbentstructure of the present invention. In general, it is desired that thebasic, water-swellable, water-insoluble polymer have a degree ofneutralization of its basic functional groups that is beneficially lessthan about 50 molar percent, more beneficially less than about 30 molarpercent, suitably less than about 20 molar percent, more suitably lessthan about 10 molar percent, and most suitably substantially about 0molar percent.

Commercially available superabsorbents are generally in a substantiallyneutralized or salt form. This is because, in general, in order to havea relatively high capacity for liquid absorption, a water-swellable,water-insoluble polymer must be a polyelectrolyte. However, as discussedherein, the basic water-swellable, water-insoluble polymers useful inthe present invention are substantially in their free base form.Therefore, such basic water-swellable, water-insoluble polymers in theirfree base form generally do not have, on their own, a relatively highcapacity for liquid absorption.

It has been discovered in the present invention, however, that when sucha basic water-swellable, water-insoluble polymer, substantially in itsfree base form, is combined or mixed with an acidic second material, theresulting combination or mixture will exhibit a relatively high capacityfor liquid absorption. This is believed to be because as the mixture isplaced in an aqueous solution, the basic water-swellable,water-insoluble polymer, substantially in its free base form, reactswith the acidic second material, and the chemical equilibrium is infavor of converting the basic water-swellable, water-insoluble polymerfrom its free base form to its respective salt form. As such, themixture comprising the substantially neutralized water-swellable,water-insoluble polymer will now exhibit a relatively high capacity forliquid absorption. In addition, the conversion of the water-swellable,water-insoluble polymer, from its free base form to its respective saltform in an electrolyte-containing solution, such as an aqueous sodiumchloride solution or urine, can have a substantial desalting effect onthe electrolyte-containing solution, thereby improving theliquid-absorbing performance of the mixture comprising thewater-swellable, water-insoluble polymer by alleviating anysalt-poisoning effect.

In contrast to the above, it has been found that a single material orpolymer, comprising both acidic and basic functional groups within itsmolecular structure, will not exhibit the desired absorbent propertiesdescribed herein. This is believed to be because such acidic and basicfunctional groups within a single molecular structure will typicallyreact with each other and might result in an over-crosslinked polymerstructure. As such, it generally is not possible to prepare theabsorbent structure of the present invention by preparing a copolymerfrom acidic and basic monomers or by preparing a molecular leveldispersion, such as in an aqueous solution, of water-soluble acidic andbasic materials since during such copolymerization or molecular leveldispersion the acidic and basic materials will typically react with eachother and crosslink.

The acidic or basic water-swellable, water-insoluble polymer maygenerally be used in the absorbent structure in a variety of forms.Examples of forms that the acidic or basic water-swellable,water-insoluble polymer may take include particles, flakes, fibers,films, and nonwoven structures. When the absorbent structure is used indisposable absorbent products, it is generally desired that the acidicor basic water-swellable, water-insoluble polymer be in the form ofdiscrete particles, fibers, or flakes in a fibrous matrix. When in theform of a particle, it is generally desired that the particle have amaximum cross-sectional dimension beneficially within the range fromabout 50 micrometers to about 2,000 micrometers, suitably within therange from about 100 micrometers to about 1,000 micrometers, and moresuitably within the range from about 300 micrometers to about 600micrometers.

When the first component to be used in the absorbent structure of thepresent invention is an acidic water-swellable, water-insoluble polymer,the second component to be used in the absorbent structure of thepresent invention is a basic material. As used herein, a “basic”material is intended to refer to a material that may act as an electrondonor and which, in an aqueous solution, exhibits a pH between 7 toabout 14. Suitably, the pH is measured at about 25° C. Examples ofsuitable basic second materials include, but are not limited to,polymeric basic materials such as polyamines, polyimines, polyamides,polyquaternary ammoniums, chitins, chitosans, polyasparagines,polyglutamines, polylysines, and polyarginines; organic basic materialssuch as organic salts such as sodium citrate and aliphatic and aromaticamines, imines, and amides; and inorganic bases such as metallic oxides,such as calcium oxide and aluminum oxide; hydroxides, such as bariumhydroxide; salts such as sodium carbonate, sodium bicarbonate, andcalcium carbonate; and mixtures thereof. The basic second material cangenerally be either a strong or a weak base. However, the strength ofthe basicity of the basic second material has been found to potentiallyaffect the liquid absorption rate of the absorbent structure. Generally,an absorbent structure comprising a relatively stronger basic secondmaterial will exhibit a relatively faster liquid absorption rate ascompared to an absorbent structure comprising a relatively weaker basicsecond material.

In general, basic second materials useful in the absorbent structure maybe either strongly or weakly basic in nature. In general, a basic secondmaterial that is strongly basic will exhibit a pKa value greater thanabout 12. In general, a basic second material that is weakly basic willexhibit a pKa that is less than about 12. As such, basic secondmaterials useful in the absorbent structure may exhibit a broad range ofpKa values, but will beneficially have a pKa between about 4 to about14, more beneficially between about 5 to about 14, and suitably betweenabout 8 to about 14.

In one beneficial embodiment of the present invention, the basic secondmaterial can also suitably be a water-swellable, water-insolublepolymer. In such an embodiment, both the acidic water-swellable,water-insoluble polymer and the basic, water-swellable, water-insolublepolymeric second material can be used to contribute to the total liquidabsorptive capacity of the absorbent structure, thereby potentiallyachieving an overall higher liquid absorptive capacity of the absorbentstructure as compared to the use of a basic second material that is nota water-swellable, water-insoluble polymer.

It may sometimes be more convenient to measure the pKa of the monomer ormonomers used to prepare a polymer. Although the pKa of the monomer ormonomers and the polymer prepared from such monomers may not beidentical, such pKa values should be substantially similar. As such,basic, water-swellable, water-insoluble polymers useful in the absorbentstructure may be prepared from a single monomer or a combination ofmonomers that exhibit a broad range of pKa values, but such monomerswill beneficially have a pKa between about 4 to about 14, morebeneficially between about 5 to about 14, and suitably between about 8to about 14.

The pKa of a base represents the extent of dissociation of or, in otherwords, the strength of the base and is intended herein to be measured atthe conditions, such as at a specific temperature, under which the baseis being used. Suitably, the pKa is measured at about 25° C. In general,the weaker the base, the lower the pKa value will be. The pKa values formany bases at various temperatures are well known and may be found inany of many available references, such as in the CRC Handbook ofChemistry & Physics, 75^(th) Edition, edited by David R. Lide, CRC Press(1994).

Suitable basic second materials will include functional groups that arecapable of acting as bases. Such functional groups include, but are notlimited to, primary, secondary, or tertiary amino groups, imino groups,imido groups, and amido groups. Suitably the functional groups are aminogroups. When the basic second material is a water-swellable,water-insoluble polymer, the functional groups are generally attached toa crosslinked base polymer. Suitable base polymers include polyamines,polyimines, polyamides, and polyquaternary ammoniums, and copolymersthereof. Natural based polysaccharide polymers may also be used andinclude chitin and chitosan. Synthetic polypeptides can also be usedsuch as polyasparagines, polyglutamines, polylysines, and polyarginines.

A basic, water-swellable, water-insoluble polymer generally needs to bein its free base form. In general, it is desired that the basic,water-swellable, water-insoluble polymer beneficially have at leastabout 50 molar percent, more beneficially at least about 70 molarpercent, suitably at least about 80 molar percent, more suitably atleast about 90 molar percent, and most suitably substantially about 100molar percent of its basic functional groups in free base form.Alternatively, then, the basic, water-swellable, water-insoluble polymershould not be substantially neutralized when used in the absorbentstructure of the present invention. In general, it is desired that thebasic, water-swellable, water-insoluble polymer have a degree ofneutralization of its basic functional groups that is beneficially lessthan about 50 molar percent, more beneficially less than about 30 molarpercent, suitably less than about 20 molar percent, more suitably lessthan about 10 molar percent, and most suitably substantially about 0molar percent.

When the first component to be used in the absorbent structure of thepresent invention is a basic water-swellable, water-insoluble polymer,the second component to be used in the absorbent structure of thepresent invention is an acidic material. As used herein, an “acidic”material is intended to refer to a material that may act as an electronacceptor and which, in an aqueous solution, exhibits a pH between about0 to 7. Suitably, the pH is measured at about 25° C.

Examples of suitable acidic second materials include, but are notlimited to, polymeric acidic materials such as polyacrylic acid,polymaleic acid, carboxymethyl cellulose, alginic acid, polyasparticacid, and polyglutamic acid; organic acidic materials such as aliphaticand aromatic acids, such as citric acid, glutamic acid, and asparticacid; inorganic acids such as metallic oxides, such as aluminum oxide;salts such as iron chloride, calcium chloride, and zinc chloride; andmixtures thereof. The acidic second material can generally be either astrong or a weak acid. However, the strength of the acidity of theacidic second material has been found to potentially affect the liquidabsorption rate of the absorbent structure. Generally, an absorbentstructure comprising a relatively stronger acidic second material willexhibit a relatively faster liquid absorption rate as compared to anabsorbent structure comprising a relatively weaker acidic secondmaterial.

In general, acidic second materials useful in the absorbent structuremay be either strongly or weakly acidic in nature. In general, an acidicsecond material that is strongly acidic will exhibit a pKa value lessthan about 2. In general, an acidic second material that is weaklyacidic will exhibit a pKa that is greater than about 2. As such, acidicsecond materials useful in the absorbent structure may exhibit a broadrange of pKa values, but will beneficially have a pKa between about 0 toabout 12, more beneficially between about 2 to about 10, and suitablybetween about 3 to about 7.

In one beneficial embodiment of the present invention, the acidic secondmaterial can also suitably be a water-swellable, water-insolublepolymer. In such an embodiment, both the basic water-swellable,water-insoluble polymer and the acidic, water-swellable, water-insolublepolymeric second material can be used to contribute to the total liquidabsorptive capacity of the absorbent structure, thereby potentiallyachieving an overall higher liquid absorptive capacity of the absorbentstructure as compared to the use of an acidic second material that isnot a water-swellable, water-insoluble polymer.

It may sometimes be more convenient to measure the pKa of the monomer ormonomers used to prepare a polymer. Although the pKa of the monomer ormonomers and the polymer prepared from such monomers may not beidentical, such pKa values should be substantially similar. As such,acidic, water-swellable, water-insoluble polymers useful in theabsorbent structure may be prepared from a single monomer or acombination of monomers that exhibit a broad range of pKa values, butsuch monomers will beneficially have a pKa between about 0 to about 12,more beneficially between about 2 to about 10, and suitably betweenabout 3 to about 7.

The pKa of an acid represents the extent of dissociation of or, in otherwords, the strength of the acid and is intended herein to be measured atthe conditions, such as at a specific temperature, under which the acidis being used. Suitably, the pKa is measured at about 25° C. In general,the weaker the acid, the higher the pKa value will be. The pKa valuesfor many bases at various temperatures are well known and may be foundin any of many available references, such as in the CRC Handbook ofChemistry & Physics, 75^(th) Edition, edited by David R. Lide, CRC Press(1994).

Suitable acidic second materials will include functional groups that arecapable of acting as acids. Such functional groups include, but are notlimited to, carboxyl groups, sulfonic groups, sulphate groups, sulfitegroups, and phosphate groups. Suitably, the functional groups arecarboxyl groups. When the acidic second material is a water-swellable,water-insoluble polymer, the functional groups are generally attached toa crosslinked base polymer. Suitable base polymers includepolyacrylamides, polyvinyl alcohols, ethylene maleic anhydridecopolymer, polyvinylethers, polyacrylamido methylpropane sulfonic acid,polyacrylic acids, polyvinylpyrrolidones, polyvinylmorpholines, andcopolymers of the preceding polymers. Natural based polysaccharidepolymers may also be used and include carboxymethyl celluloses,carboxymethyl starches, hydroxypropyl celluloses, algins, alginates,carrageenans, acrylic grafted starches, acrylic grafted celluloses, andcopolymers of the preceding polymers. Synthetic polypeptides can also beused such as polyaspartic acid and polyglutamic acid.

An acidic, water-swellable, water-insoluble polymer generally needs tobe in its free acid form. In general, it is desired that the acidic,water-swellable, water-insoluble polymer beneficially have at leastabout 50 molar percent, more beneficially at least about 70 molarpercent, suitably at least about 80 molar percent, more suitably atleast about 90 molar percent, and most suitably substantially about 100molar percent of its acidic functional groups in free acid form.Alternatively, then, the acidic, water-swellable, water-insolublepolymer should not be substantially neutralized when used in theabsorbent structure of the present invention. In general, it is desiredthat the acidic, water-swellable, water-insoluble polymer have a degreeof neutralization of its acidic functional groups that is beneficiallyless than about 50 molar percent, more beneficially less than about 30molar percent, suitably less than about 20 molar percent, more suitablyless than about 10 molar percent, and most suitably substantially about0 molar percent.

The basic or acidic second material may generally be used in theabsorbent structure in a variety of forms. Examples of forms that thebasic or acidic second material may take include particles, flakes,fibers, films, and nonwoven structures. When the absorbent structure isused in disposable absorbent products, it is generally desired that thebasic or acidic second material be in the form of discrete particles,fibers, or flakes in a fibrous matrix. When in the form of a particle,it is generally desired that the particle have a maximum cross-sectionaldimension beneficially within the range from about 50 micrometers toabout 2,000 micrometers, suitably within the range from about 100micrometers to about 1,000 micrometers, and more suitably within therange from about 300 micrometers to about 600 micrometers. Thecombination of the acidic water-swellable, water-insoluble polymer andthe basic second material, or of the basic water-swellable,water-insoluble polymer and the acidic second material, may also be inthe form of bicomponent fibers, wherein one component is the acidic orbasic water-swellable, water-insoluble polymer and the other componentis the basic or acidic second material. Such a bicomponent fiber may bea side-by-side bicomponent fiber or a sheath-and-core bicomponent fiber.Such bicomponent fibers may be prepared by known methods, such asco-extrusion methods.

In general, the acidic water-swellable, water-insoluble polymer,substantially in its free acid form, is mixed with a basic secondmaterial in the absorbent structure in a molar ratio of the respectiveacidic and basic functionalities that is sufficient to provide theabsorbent structure with desired absorbent and pH properties. The molarratio of the acidic water-swellable, water-insoluble polymer to thebasic second material is beneficially from about 10:1 to about 1:10,suitably from about 4:1 to about 1:4, more suitably from about 2:1 toabout 1:2, and most suitably at about 1:1.

In general, the basic water-swellable, water-insoluble polymer,substantially in its free base form, is mixed with an acidic secondmaterial in the absorbent structure in a molar ratio of the respectivebasic and acidic functionalities that is sufficient to provide theabsorbent structure with desired absorbent and pH properties. The molarratio of the basic water-swellable, water-insoluble polymer to theacidic second material is beneficially from about 10:1 to about 1:10,suitably from about 4:1 to about 1:4, more suitably from about 2:1 toabout 1:2, and most suitably at about 1:1.

In one embodiment of the present invention, the acidic or basicwater-swellable, water-insoluble polymer and the basic or acidic secondmaterial may be mixed together to prepare an absorbent composition. Anabsorbent composition comprising the acidic or basic water-swellable,water-insoluble polymer and the basic or acidic second material suitablyhas the ability to absorb a liquid, herein referred to as Free Swell(FS). The method by which the Free Swell value is determined is setforth below in connection with the examples. The Free Swell valuesdetermined as set forth below and reported herein refer to the amount ingrams of an aqueous solution, containing 0.9 weight percent sodiumchloride, a gram of a material can absorb in about 10 hours under anegligible load of about 0.01 pound per square inch (psi). As a generalrule, it is desired that an absorbent composition has a Free Swellvalue, for a load of about 0.01 psi, of at least about 15, beneficiallyof at least about 20, suitably of at least about 25, and up to about 200grams per gram.

An absorbent composition comprising the acidic or basic water-swellable,water-insoluble polymer and the basic or acidic second material alsosuitably has the ability to absorb a liquid while the absorbentcomposition is under an external pressure or load, herein referred to asAbsorbency Under Load (AUL). Synthetic polymeric materials, such assodium polyacrylates, having a generally high ability to absorb a liquidwhile under a load, have been found to minimize the occurrence ofgel-blocking when incorporated in absorbent products. The method bywhich the Absorbency Under Load is determined is set forth below inconnection with the examples. The Absorbency Under Load valuesdetermined as set forth below and reported herein refer to the amount ingrams of an aqueous solution, containing 0.9 weight percent sodiumchloride, a gram of a material can absorb in about 10 hours under a loadof about 0.3 pound per square inch (psi). As a general rule, it isdesired that an absorbent composition has an Absorbency Under Loadvalue, for a load of about 0.3 psi, of at least about 15, beneficiallyof at least about 20, suitably of at least about 25, and up to about 100grams per gram.

In one embodiment of the present invention, an absorbent compositioncomprising the acidic or basic water-swellable, water-insoluble polymerand the basic or acidic second material suitably has the ability torelatively slowly absorb a liquid. The use of acidic, water-swellable,water-insoluble polymers that are strongly acidic, exhibiting a pKa lessthan about 2, or the use of basic, water-swellable, water-insolublepolymers that are strongly basic, exhibiting a pKa greater than about12, have been found to generally result in absorbent compositions thatgenerally do not exhibit the desired slow-rate of absorbing liquids. Theuse of acidic, water-swellable, water-insoluble polymers that are tooweakly acidic, exhibiting a pKa greater than about 12, or the use ofbasic, water-swellable, water-insoluble polymers that are weakly basic,exhibiting a pKa less than about 2, have generally been found to resultin absorbent compositions that generally do not exhibit the desiredliquid absorbent capacity. The preparation of an absorbent compositioncomprising, in one embodiment, an acidic water-swellable,water-insoluble polymer and a basic second material or, in anotherembodiment, a basic water-swellable, water-insoluble polymer and anacidic second material, and suitably having the ability to relativelyslowly absorb a liquid is described in copending U.S. patent applicationSer. No. 08/759,108, filed Dec. 2, 1996, the specification of which ishereby incorporated in its entirety.

As used herein, the quantification of the rate with which an absorbentcomposition absorbs a liquid will be referred to as the Time to Reach 60Percent of Free Swell Capacity value. The method by which the Time toReach 60 Percent of Free Swell Capacity value is determined is set forthbelow in connection with the examples. The Time to Reach 60 Percent ofFree Swell Capacity values determined as set forth below and reportedherein refer to the time, in minutes, that it takes an absorbentcomposition to absorb about 60 percent of the absorbent composition'stotal absorptive capacity, as represented by the absorbent composition'sFree Swell value. In one embodiment, it is desired that the absorbentcomposition of the present invention has a Time to Reach 60 Percent ofFree Swell Capacity value of at least about 5 minutes, beneficiallybetween about 5 minutes to about 300 minutes, more beneficially betweenabout 10 minutes to about 200 minutes, suitably between about 20 minutesto about 100 minutes, and more suitably between about 30 minutes toabout 60 minutes.

In another embodiment, an absorbent composition of the present inventionsuitably also has the ability to relatively slowly absorb a liquid whilethe absorbent composition is under an external pressure or load. As usedherein, the quantification of the rate with which an absorbentcomposition absorbs a liquid while the absorbent composition is under anexternal pressure or load will be referred to as the Time to Reach 60Percent of Absorbency Under Load Capacity value. The method by which theTime to Reach 60 Percent of Absorbency Under Load Capacity value isdetermined is set forth below in connection with the examples. The Timeto Reach 60 Percent of Absorbency Under Load Capacity values determinedas set forth below and reported herein refer to the time, in minutes,that it takes an absorbent composition to absorb about 60 percent of theabsorbent composition's total absorptive capacity under an externalpressure or load, as represented by the absorbent composition'sAbsorbency Under Load value. In such an embodiment, it is desired thatthe absorbent composition has a Time to Reach 60 Percent of AbsorbencyUnder Load Capacity value of at least about 5 minutes, beneficiallybetween about 5 minutes to about 300 minutes, more beneficially betweenabout 10 minutes to about 200 minutes, suitably between about 20 minutesto about 100 minutes, and more suitably between about 30 minutes toabout 60 minutes.

Although an absorbent structure comprising, in one embodiment, theacidic water-swellable, water-insoluble polymer and the basic secondmaterial or, in another embodiment, a basic water-swellable,water-insoluble polymer and an acidic second material, may exhibit adesired liquid absorption capacity, it has been discovered as part ofthe present research that if either the difference in dissociationconstants between the acidic or basic water-swellable, water-insolublepolymer and the basic or acidic second material is too large or thedifference in solubility or dispersibility in an aqueous solutionbetween the acidic or basic water-swellable, water-insoluble polymer andthe basic or acidic second material is too large, then a sufficientlylarge temporary imbalance in the number of dissociated ions from theacidic or basic water-swellable, water-insoluble polymer and the basicor acidic second material will occur and cause a temporary pH imbalance,resulting in the absorbent structure exhibiting a pH that may beundesirably too high or too low. If the pH value within the absorbentstructure, particularly at or along the upper surface of the absorbentstructure oriented toward the skin of the wearer or user, is allowed toreach too high of or too low of pH values, then such an absorbentstructure may result in or increase the chances of a wearer or userexperiencing skin irritation. Thus, it is desired that the upper surfaceof the absorbent structure oriented toward the skin of the wearermaintain a substantially desired and balanced pH profile while theabsorbent structure is being worn or used.

In general, it is desired that the upper surface of the absorbentstructure oriented toward the skin of the wearer, generally along theentire length and width of the upper surface of the absorbent structure,exhibits a pH that remains beneficially between about 3 to about 8, morebeneficially between about 4 to about 7, and suitably between about 5 toabout 6.

As discussed above, it has been discovered that an imbalance in thenumber of dissociated ions from the acidic or basic water-swellable,water-insoluble polymer and the basic or acidic second material mayoccur due to at least two different characteristics of the respectivematerials. First, too large of a difference in the dissociationconstants of the acidic or basic water-swellable, water-insolublepolymer and the basic or acidic second material may result inundesirable pH values occurring within an absorbent structure. Such asituation may result, for example, from the use of a strongly acidic orbasic water-swellable, water-insoluble polymer and a weakly basic oracidic second material or, alternatively, from the use of a weaklyacidic or basic water-swellable, water-insoluble polymer and a stronglybasic or acidic second material. In general, a strongly acidic or basicmaterial has the capability of reaching a more complete ionization in anaqueous solution whereas a weakly acidic or basic material can generallyonly reach a partial ionization in an aqueous solution.

Second, too large of a difference in the solubility or dispersibilitycharacteristics in an aqueous solution between the acidic or basicwater-swellable, water-insoluble polymer and the basic or acidic secondmaterial may also result in undesirable pH values occurring within anabsorbent structure. In such a situation, the more soluble ordispersible material can reach its ionization equilibrium more quicklywhereas the less soluble or dispersible material will generally take alonger time to reach its ionization equilibrium.

Thus, differences in the dissociation constants as well as thesolubility or dispersibility of the acidic and basic components canresult in an imbalance in pH. In order to measure or quantify thisimbalance, a method to measure the ionization rate of acidic and basiccomponents has been developed. The ionization rate of a component asdescribed herein represents a combination of several factors such as thedissociation constant and the solubility or dispersibility value of thecomponent, the ionic content of a liquid in which the component isplaced, and other conditions of use. By measuring the ionization rate ofa component, it has been found possible to device methods that wouldenable the imbalance in pH in an absorbent structure to be minimized andbrought within the generally acceptable range for skin-wellness. Variousphysical approaches that do not require additional chemical species suchas buffering agents can be used to achieve a balanced pH on the surfaceof an absorbent structure. The following are examples of some of thephysical approaches that could be used to overcome the problem ofmaintaining the pH within a desired range at the surface of an absorbentstructure.

One method of attaining a balanced pH profile is to balance theionization rates of the acidic and basic components by using the acidicand basic components with an appropriate particle size range. Theparticle size of a component is generally inversely related to surfacearea of the component. As such, using a smaller particle size for acomponent with a relatively slower ionization rate will generally exposea larger surface area of the component to a liquid. As dissolution andionization of a component occurs only when the component contacts aliquid, providing a larger surface area over which this contact withliquid can be made generally ensures a faster ionization rate for thecomponent. In this approach, the particle size for a component with arelatively faster ionization rate would be relatively larger as comparedto the particle size of a component with a relatively slower ionizationrate. By carefully selecting the particle size of the acidic and basiccomponents to be used in an absorbent structure, this approach enablesthe ionization rates of the acidic and basic components to beeffectively matched, thereby generally resulting in a balanced pHprofile, particularly on the upper surface of an absorbent structure.Other approaches to effectively control the surface area of thecomponents can also be used as, for example, by using components havingdifferent shapes or morphologies.

Another approach that has been found to effectively balance theionization rates of different components is to coat or encapsulateanother substance onto the surface of the acidic and/or basiccomponents. By coating or encapsulating a component, the ionization rateof the component can generally be slowed down due to the diffusionbarrier created by the coating or encapsulating material. For example,this approach can be used to coat or encapsulate a component having arelatively faster ionization rate in order to make such a componentcompatible with a component having a relatively slower ionization rate.In one embodiment of this approach, the component with the relativelyslower ionization rate could be used as the coating or encapsulatingmaterial onto the component with the relatively faster ionization rate.

Yet another approach that has been found is the physical separation ofthe acidic and basic components so as to ensure that the pH on thesurface of an absorbent structure remains within a desired range.Examples of this approach could include, but are not limited to, using abarrier material to separate the components or zoning the components ina layered structure. In general, this approach ensures strategicplacement of the components so that the ions from the component with therelatively faster ionization rate require a longer time to reach theupper surface of an absorbent structure and the ions from the componentwith the relatively slower ionization rate requires a shorter time. Thisensures that the pH at the upper surface of an absorbent structure ismaintained within a desired range.

Still another approach that has been found is the use of an acidiccomponent which is comprised of a mixture of acidic materials havingdifferent ionization rates. For example, a mixture of polyacrylic acidparticles with different degrees of neutralization could be used suchthat the resulting ionization rate for this mixture effectively matchesthe ionization rate of the basic second material being used. Thisapproach could also be achieved by using materials with a shell-corestructure which has different degrees of neutralization in the shell andthe core. A similar approach can also be applied for the basiccomponent.

In one embodiment of the present invention, a third component is used inthe absorbent structure of the present invention wherein the thirdcomponent is a buffering agent. As used herein, the term “bufferingagent” is intended to represent a chemical material or materials, or thecorresponding acid or base of such material or materials, that exhibitsa pKa between about 2 to about 10. A buffering agent, when in an aqueoussolution, generally results in such solution exhibiting only slight pHchanges on the addition of an acid or a base to the solution. Such abuffering agent therefore minimizes changes in the hydrogen ionconcentration in an aqueous solution which would otherwise tend to occuras a result of an imbalance in the ionization of any acid or basepresent in the aqueous solution.

In the present invention, the selection of an effective buffering agentis generally dependent upon the strength and solubility of each of theacidic or basic water-swellable, water-insoluble polymer and the basicor acidic second material being used in an absorbent structure. Forexample, when the acidic water-swellable, water-insoluble polymer isweakly acidic and the basic second material is strongly basic but iseither soluble or insoluble, then the buffering agent will generallyneed to be an acidic buffering agent. When the acidic water-swellable,water-insoluble polymer is weakly acidic and the basic second materialis weakly basic and is soluble, then the buffering agent will generallyneed to be an acidic buffering agent. When the acidic water-swellable,water-insoluble polymer is strongly acidic and the basic second materialis weakly basic and is insoluble, then the buffering agent willgenerally need to be a basic buffering agent. When the basicwater-swellable, water-insoluble polymer is weakly basic and the acidicsecond material is strongly acidic but is either soluble or insoluble,then the buffering agent will generally need to be a basic bufferingagent. When the basic water-swellable, water-insoluble polymer is weaklybasic and the acidic second material is weakly acidic and is soluble,then the buffering agent will generally need to be a basic bufferingagent. When the basic water-swellable, water-insoluble polymer isstrongly basic and the acidic second material is weakly acidic and isinsoluble, then the buffering agent will generally need to be an acidicbuffering agent.

When a single acid or base is used as a buffering agent, the range ofthe buffering effect of such a buffering agent is generallyapproximately one pH unit on either side of the pKa of the bufferingagent. For example, citric acid has a pKa₁ of about 3.2 and generallyresults in a buffering solution having a pH range of between about 2 toabout 4.5. Ammonia has a pKa of about 9.2 and generally results in abuffering solution having a pH range of between about 8.2 to about 10.2.When there are two or more acidic or basic groups per molecule, or amixture of several buffering agents is used, the pH range of thebuffering solution is generally larger. For example, a mixture of citricacid and dibasic sodium phosphate results in a buffering agent solutionhaving a pH range of between about 2.2 to about 8.0. As another example,a mixture of monobasic potassium phosphate and dibasic sodium phosphateresults in a buffering agent solution having a pH range of between about6.1 to about 7.5. As another example, a mixture of sodium hydroxide anddibasic sodium phosphate results in a buffering agent solution having apH range of between about 11.0 to about 12.0.

When an acidic buffering agent is desired to be used in the presentinvention, suitable buffering agents are generally acids, or the saltsof such acids, having a pKa between about 2 to about 7. Such acidsinclude, but are not limited to aspartic acid (having a pKa₁ of about3.86), ascorbic acid (having a pKa₁ of about 4.10), chloroacetic acid(having a pKa of about 2.85), β-chlorobutyric acid (having a pKa ofabout 4.05), cis-cinnamic acid (having a pKa of about 3.89), citric acid(having a pKa₁ of about 3.14), fumaric acid (having a pKa₁ of about3.03), glutaramic acid (having a pKa of about 4.60), glutaric acid(having a pKa₁ of about 4.31), itaconic acid (having a pKa₁ of about3.85), lactic acid (having a pKa of about 3.08), malic acid (having apKa₁ of about 3.40), malonic acid (having a pKa₁ of about 2.83),o-phthalic acid (having a pKa₁ of about 2.89), succinic acid (having apKa₁ of about 4.16), α-tataric acid (having a pKa₁ of about 2.89), andphosphoric acid (having a pKa₁ of about 2.12).

When a basic buffering agent is desired to be used in the presentinvention, suitable buffering agents are generally bases, or the saltsof such bases, having a pKa between about 5 to about 10. Such basesinclude, but are not limited to α-alanine (having a pKa of about 9.87),allantoin (having a pKa₁ of about 8.96), cysteine (having a pKa of about7.85), cystine (having a pKa of about 7.85), dimethylglycine (having apKa of about 9.89), histidine (having a pKa of about 9.17), glycine(having a pKa of about 9.78), chitosan (having a pKa of about 7),N-(2-acetamido)-2-iminodiacetic acid (having a pKa of about 6.8),tris(hydroxymethyl)aminomethane (having a pKa of about 8.1), theobromine(having a pKa of about 7.89), and tyrosine (having a pKa of about 8.40).

The buffering agent may generally be used in the absorbent structure ina variety of forms. Examples of forms that the buffering agent may takeinclude particles, flakes, fibers, films, and nonwoven structures. Whenthe absorbent structure is used in disposable absorbent products, it isgenerally desired that the buffering agent be in the form of discreteparticles, fibers, or flakes in a fibrous matrix. When in the form of aparticle, it is generally desired that the particle have a maximumcross-sectional dimension beneficially within the range from about 50micrometers to about 2,000 micrometers, suitably within the range fromabout 100 micrometers to about 1,000 micrometers, and more suitablywithin the range from about 300 micrometers to about 600 micrometers.

The amount of buffering agent used in the absorbent structure of thepresent invention is generally dependent on a variety of factorsincluding the strength of the acidity or basicity of the acidic or basicwater-swellable, water-insoluble polymer, the strength of the basicityor acidity of the basic or acidic second material, the relativesolubilities of each of the acidic or basic water-swellable,water-insoluble polymer and the basic or acidic second material, the pKaof the buffering agent used, and the pH range desired to be maintainedwithin the absorbent structure. In general, the amount of bufferingagent used in the absorbent structure is such that the molar ratiobetween the acidic or basic water-swellable, water-insoluble polymer andthe buffering agent is beneficially between about 50:1 to about 2:1,more beneficially between about 40:1 to about 4:1, suitably betweenabout 30:1 to about 6:1, and more suitably between about 20:1 to about10:1. In general, the amount of buffering agent used in the absorbentstructure is such that the molar ratio between the basic or acidicsecond material and the buffering agent is beneficially between about50:1 to about 2:1, more beneficially between about 40:1 to about 4:1,suitably between about 30:1 to about 6:1, and more suitably betweenabout 20:1 to about 10:1.

In one embodiment of the present invention, the buffering agent used canbe the same material used as either the acidic water-swellable,water-insoluble polymer or the basic second material when the acidicwater-swellable, water-insoluble polymer has a pKa between about 2 toabout 7 and an acidic buffering agent is required, or when the basicsecond material has a pKa between about 5 to about 10 and a basicbuffering agent is required. For example, when polyacrylic acid is usedas the acidic water-swellable, water-insoluble polymer and sodiumhydrogen carbonate is used as the basic second material, an acidicbuffering agent, such as citric acid, is generally required in order tomaintain the pH profile in a desirable range because sodium hydrogencarbonate is more soluble than the polyacrylic acid. However,polyacrylic acid can also be used as an acidic buffering agent since thepolyacrylic acid has a pKa of about 4.25. Another example is whereinpolyacrylamide methylpropane sulfonic acid is used as the acidicwater-swellable, water-insoluble polymer and chitosan is used as thebasic second material. In this example, a basic buffering agent, such assodium hydrogen carbonate, is generally required in order to maintainthe pH profile in a desirable range because the polyacrylamidemethylpropane sulfonic acid is a strongly acidic polymer and thechitosan is a weakly basic polymer. However, chitosan can also be usedas a basic buffering agent since the chitosan has a pKa of about 7.

In another embodiment of the present invention, the buffering agent usedcan be the same material used as either the basic water-swellable,water-insoluble polymer or the acidic second material when the basicwater-swellable, water-insoluble polymer has a pKa between about 7 toabout 12 and a basic buffering agent is required, or when the acidicsecond material has a pKa between about 4 to about 9 and an acidicbuffering agent is required. For example, when chitosan is used as thebasic water-swellable, water-insoluble polymer and citric acid is usedas the acidic second material, a basic buffering agent, such as sodiumhydrogen carbonate, is generally required in order to maintain the pHprofile in a desirable range because citric acid is more soluble thanthe chitosan. However, chitosan can also be used as a basic bufferingagent since the chitosan has a pKa of about 7. Another example iswherein crosslinked polydiallyl dimethyl ammonium hydroxide is used asthe basic water-swellable, water-insoluble polymer and crosslinkedpolyacrylic acid is used as the acidic second material. In this example,an acidic buffering agent, such as citric acid, is generally required inorder to maintain the pH profile in a desirable range because thepolydiallyl dimethyl ammonium hydroxide is a strongly basic polymer andthe polyacrylic acid is a weakly acidic polymer. However, polyacrylicacid can also be used as an acidic buffering agent since the polyacrylicacid has a pKa of about 4.25.

In one embodiment of the present invention, it is desired that theacidic or basic water-swellable, water-insoluble polymer, the basic oracidic second material, and, optionally, the buffering agent be preparedas an absorbent composition that may be incorporated into an absorbentstructure. Such an absorbent composition may be prepared by a simpleprocess. In general, the method of making such an absorbent compositioncomprises the step of mixing together the acidic or basicwater-swellable, water-insoluble polymer, the basic or acidic secondmaterial, and, optionally, the buffering agent.

When the second basic material is water-insoluble, such as crosslinkedpolydiallyl dimethyl ammonium hydroxide, homogeneous mixing of theacidic water-swellable, water-insoluble polymer with the basic secondmaterial is generally required for promoting uniform ion exchanging andachieving a desirable pH profile. However, when the second basicmaterial is water-soluble, such as sodium hydrogen carbonate,homogeneous mixing of the acidic water-swellable, water-insolublepolymer with the basic second material is generally not required due tothe mobility of the basic second material when the absorbent structureis insulted with a liquid. The basic second material can dissolve intoand flow with the liquid to reach the acidic water-swellable,water-insoluble polymer.

When the acidic second material is water-insoluble, such as crosslinkedpolyacrylic acid, homogeneous mixing of the basic water-swellable,water-insoluble polymer with the acidic second material is generallyrequired for promoting uniform ion exchanging and achieving a desirablepH profile. However, when the second acidic material is water-soluble,such as citric acid, homogeneous mixing of the basic water-swellable,water-insoluble polymer with the acidic second material is generally notrequired due to the mobility of the acidic second material when theabsorbent structure is insulted with a liquid. The acidic secondmaterial can dissolve into and flow with the liquid to reach the basicwater-swellable, water-insoluble polymer.

Mixtures of the components should generally be prepared under conditionsthat are sufficient for the acidic or basic water-swellable,water-insoluble polymer, the basic or acidic second material, and,optionally, the buffering agent to be effectively mixed together. Suchmixtures will beneficially be agitated, stirred, or otherwise blended toeffectively mix the acidic or basic water-swellable, water-insolublepolymer, the basic or acidic second material, and, optionally, thebuffering agent such that an essentially uniform mixture is formed.Equipment for achieving such agitation, stirring, or blending are wellknown in the art and include simple blenders and mixers and suitableforming equipment.

In another embodiment of the present invention, the buffering agent maybe used as or contained in an essentially separate layer or component ofthe absorbent structure such as, for example, a surge layer or a tissuesheet that is located near the upper surface of the absorbent structure.Such an embodiment may be effective in reducing the amount of bufferingagent that is needed in the absorbent structure in order to achieve thedesired property of maintaining a substantially desired and balanced pHprofile on or along the upper surface of the absorbent structure. Inaddition, substantially separating the acidic or basic water-swellable,water-insoluble polymer and the buffering agent may help to enhance thetotal liquid absorbing capacity of the absorbent structure due to beingable to maintain the acidic or basic water-swellable, water-insolublepolymer at a relatively higher pH which may increase the liquidabsorbing capacity of the acidic or basic water-swellable,water-insoluble polymer.

While the principal components of the absorbent structure have beendescribed in the foregoing, such an absorbent structure is not limitedthereto and can include other components not adversely effecting theabsorbent structure or the use of the absorbent structure having thedesired absorbent and pH properties. Exemplary materials which could beused as additional components would include, without limitation,pigments, antioxidants, stabilizers, surfactants, waxes, flow promoters,solid solvents, particulates, and materials added to enhance theprocessability of the absorbent structure.

The absorbent structure of the present invention is suitable for use indisposable absorbent products such as personal care products, such asdiapers, training pants, baby wipes, feminine care products, adultincontinent products; medical products, such as wound dressings orsurgical capes or drapes; and tissue products. In one embodiment of thepresent invention, a disposable absorbent product is provided, whichdisposable absorbent product comprises a liquid-permeable topsheet, abacksheet attached to the topsheet, and an absorbent structurepositioned between the topsheet and the backsheet wherein the absorbentstructure comprises the acidic water-swellable, water-insoluble polymer,the basic second material, and the buffering agent, and wherein theabsorbent structure exhibits desired absorbent and pH properties.

Disposable absorbent products, according to all aspects of the presentinvention, are generally subjected during use to multiple insults of abody liquid. Accordingly, the disposable absorbent products aredesirably capable of absorbing multiple insults of body liquids inquantities to which the absorbent products and structures will beexposed during use. The insults are generally separated from one anotherby a period of time.

Those skilled in the art will recognize materials suitable for use asthe topsheet and backsheet. Exemplary of materials suitable for use asthe topsheet are liquid-permeable materials, such as spunbondedpolypropylene or polyethylene having a basis weight of from about 15 toabout 25 grams per square meter. Exemplary of materials suitable for useas the backsheet are liquid-impervious materials, such as polyolefinfilms, as well as vapor-pervious materials, such as microporouspolyolefin films.

The acidic or basic water-swellable, water-insoluble polymer, the basicor acidic second material, and, optionally, the buffering agent aretypically present in an absorbent structure in conjunction with afibrous matrix. A fibrous matrix may take the form of, for example, abatt of comminuted wood pulp fluff, a tissue layer, a hydroentangledpulp sheet, or a mechanically softened pulp sheet. Suitably, the fibrousmatrix is formed so as to constrain or entrap the acidic or basicwater-swellable, water-insoluble polymer, the basic or acidic secondmaterial, and, optionally, the buffering agent within, or onto, itsstructure. The acidic or basic water-swellable, water-insoluble polymer,the basic or acidic second material, and, optionally, the bufferingagent may be incorporated into or onto the fibrous matrix either duringor after the formation of the general form of the fibrous matrix. Afibrous matrix useful in the present invention may be formed by anair-laying process or a wet-laid process, or by essentially any otherprocess known to those skilled in the art for forming a fibrous matrix.The fibrous matrix may be formed from either natural fibers or syntheticfibers or a mixture of both natural and synthetic fibers.

The acidic or basic water-swellable, water-insoluble polymer, the basicor acidic second material, and, optionally, the buffering agent aretypically present in an absorbent structure or disposable absorbentproduct of the present invention in an amount effective to result in theabsorbent structure or disposable absorbent product being able to absorba desired amount of liquid and exhibit desired pH properties. The acidicor basic water-swellable, water-insoluble polymer, the basic or acidicsecond material, and, optionally, the buffering agent are beneficiallypresent in an absorbent structure in an amount of from about 1 to about100 weight percent, more beneficially in an amount of from about 5 toabout 95 weight percent, suitably in an amount of from about 10 to about90 weight percent, and more suitably of from about 30 to about 70 weightpercent, based on the total weight of the absorbent structure.

It is generally desired that the absorbent structure of the presentinvention has the ability to absorb a desired quantity of liquid, suchas urine, blood, menses, synthetic urine, or an aqueous solutioncomprising 0.9 weight percent sodium chloride. In one embodiment of thepresent invention, it is desired that an absorbent structure has theability to absorb a quantity of liquid as quantified by a WickingCapacity value. As used herein, the Wicking Capacity value, reported ingrams per gram, refers to the amount of an aqueous solution, containing0.9 weight percent sodium chloride, a gram of an absorbent structure canabsorb in about 6 hours as measured by the method described in the TestMethods section herein.

It is generally desired that an absorbent structure exhibit a WickingCapacity value that is beneficially at least about 5 grams per gram,more beneficially at least about 10 grams per gram, suitably at leastabout 15 grams per gram, more suitably at least about 20 grams per gram,and up to about 40 grams per gram.

Test Methods

Free Swell Capacity and Time to Reach 60 Percent of Free Swell Capacity

The Free Swell Capacity (FS) is a test which measures the amount ingrams of an aqueous solution, containing 0.9 weight percent sodiumchloride, a gram of a material can absorb in 10 hours under a negligibleapplied load or restraining force, such as of about 0.01 pound persquare inch.

Referring to FIG. 1, the apparatus and method for determining the FreeSwell and the Absorbency Under Load will be described. Shown is aperspective view of the apparatus in position during a test. Shown is alaboratory jack 1 having an adjustable knob 2 for raising and loweringthe platform 3. A laboratory stand 4 supports a spring 5 connected to amodified thickness meter probe 6, which passes through the housing 7 ofthe meter, which is rigidly supported by the laboratory stand. A plasticsample cup 8, which contains the superabsorbent material sample to betested, has a liquid-permeable bottom and rests within a Petri dish 9which contains the saline solution to be absorbed. For the determinationof Absorbency Under Load values only, a weight 10 rests on top of aspacer disc (not visible) resting on top of the superabsorbent materialsample (not visible).

The sample cup consists of a plastic cylinder having a 1 inch insidediameter and an outside diameter of 1.25 inches. The bottom of thesample cup is formed by adhering a 100 mesh metal screen having 150micron openings to the end of the cylinder by heating the screen abovethe melting point of the plastic and pressing the plastic cylinderagainst the hot screen to melt the plastic and bond the screen to theplastic cylinder.

The modified thickness meter used to measure the expansion of the samplewhile absorbing the saline solution is a Mitutoyo Digimatic Indicator,IDC Series 543, Model 543-180, having a range of 0-0.5 inch and anaccuracy of 0.00005 inch (Mitutoyo Corporation, 31-19, Shiba 5-chome,Minato-ku, Tokyo 108, Japan). As supplied from Mitutoyo Corporation, thethickness meter contains a spring attached to the probe within the meterhousing. This spring is removed to provide a free-falling probe whichhas a downward force of about 27 grams. In addition, the cap over thetop of the probe, located on the top of the meter housing, is alsoremoved to enable attachment of the probe to the suspension spring 5(available from McMaster-Carr Supply Co., Chicago, Ill., Item No.9640K41), which serves to counter or reduce the downward force of theprobe to about 1 gram±0.5 gram. A wire hook can be glued to the top ofthe probe for attachment to the suspension spring. The bottom tip of theprobe is also provided with an extension needle (Mitutoyo Corporation,Part No. 131279) to enable the probe to be inserted into the sample cup.

To carry out the test, a 0.160 gram sample of an absorbent materialsample, which has typically been sieved to a particle size between 300and 600 microns, is placed into the sample cup. The sample is thencovered with a plastic spacer disc, weighing 4.4 grams and having adiameter of about 0.995 inch, which serves to protect the sample frombeing disturbed during the test and also to uniformly apply a load onthe entire sample. The sample cup, with material sample and spacer disc,is then weighed to obtain its dry weight. The sample cup is placed inthe Petri dish on the platform and the laboratory jack raised up untilthe top side of the plastic spacer disc contacts the tip of the probe.The meter is zeroed. A sufficient amount of saline solution is added tothe Petri dish (50-100 milliliters) to begin the test. The distance theplastic spacer disc is raised by the expanding sample as it absorbs thesaline solution is measured by the probe. This distance, multiplied bythe cross-sectional area inside the sample cup, is a measure of theexpansion volume of the sample due to absorption. Factoring in thedensity of the saline solution and the weight of the sample, the amountof saline solution absorbed is readily calculated. The weight of salinesolution absorbed after about 10 hours is the Free Swell value expressedas grams saline solution absorbed per gram of absorbent. If desired, thereadings of the modified thickness meter can be continuously inputted toa computer (Mitutoyo Digimatic Miniprocessor DP-2 DX) to make thecalculations and provide Free Swell readings. As a cross-check, the FreeSwell can also be determined by determining the weight differencebetween the sample cup before and after the test, the weight differencebeing the amount of solution absorbed by the sample.

From the continuous monitoring of the Free Swell values provided by thecomputer, the Time to Reach 60 Percent of Free Swell Capacity is readilydetermined.

Absorbency Under Load Capacity and Time to Reach 60 Percent ofAbsorbency Under Load Capacity

The Absorbency Under Load (AUL) is a test which measures the amount ingrams of an aqueous solution, containing 0.9 weight percent sodiumchloride, a gram of a material can absorb in 10 hours under an appliedload or restraining force of about 0.3 pound per square inch. Theprocedure for measuring the Absorbency Under Load value of an absorbentcomposition is essentially identical to the procedure for measuring theFree Swell values, except that a 100 gram weight is placed on top of theplastic spacer disc, thereby applying a load of about 0.3 pound persquare inch onto the absorbent composition as it absorbs the salinesolution. From the continuous monitoring of the Absorbency Under Loadvalues provided by the computer, the Time to Reach 60 Percent ofAbsorbency Under Load Capacity is readily determined.

Wicking Capacity and pH Range Measurements

Referring to the attached FIG. 2, the apparatus and method fordetermining the wicking capacity and pH profiles will be furtherdescribed.

FIG. 2 is an exploded perspective view of the apparatus used forcarrying out the wicking capacity and pH profile measurement. FIG. 2illustrates the test container 60 comprising a holding chamber 61, atesting chamber 62, and a cover 63. Testing chamber 62 is a rectangularchamber 5.08 cm (2 inches) wide, 35.56 cm (14 inches) long, and 4.445 cm(1.75 inches) deep (internal dimensions). The testing chamber 62 issuitably formed from a clear material such as an acrylic resincommercially available under the designation LUCITE™ (0.635 cm [0.25inch] thick). The top 64 of testing chamber 62 is open. The bottom 65 oftesting chamber 62 is formed from 100 mesh stainless steel screen. Themetal screen is adhered to the material forming the sides and ends oftesting chamber 62. The longitudinal end 66 of the test chamber 62 isformed by a piece of acrylic resin which is dimensioned such that thetesting chamber 62 defines a 5.08 cm (2 inches) wide by 0.9525 cm (0.375inch) deep opening 67 is covered with a 100 mesh stainless metal screen68. The mesh screen 68 is suitably adhered to the acrylic resin formingtest chamber 62 around the periphery of opening 67. The bottom 65 andend screen 68 are adhered at their juncture or are formed as a single,integral piece.

The holding chamber 61 comprises longitudinal ends 70, 71, lateral sides72, 73, and bottom 74. Holding chamber 61 is suitably formed from aclear resin such as acrylic resin (0.635 cm [0.25 inch] thick).Longitudinal ends 70, 71, lateral sides 72, 73, and bottom 74 of holdingchamber 61 define a top opening 75. When the testing chamber 62 isformed from 0.635 cm (0.25 inch) thick acrylic resin, the holdingchamber 61 is dimensioned to form a chamber 6.35 cm (2.5 inches) wide,36.83 cm (14.5 inches) long and 5.08 cm (2 inches) deep (internaldimensions). In any event, holding chamber 61 is internally dimensionedso that testing chamber 62 can just pass into, and snugly fit within,the interior of holding chamber 61.

Cover 63 is similarly formed from a clear acrylic resin and isdimensioned to cover the top opening 75 of the holding chamber 61 whenthe testing chamber 62 is present therein. Cover 63 defines an interiorchamber 6.35 cm (2.5 inches) wide, 36.83 cm (14.5 inches) long and1.4288 cm (0.5625 inch) deep. On the top of cover 63, there are sixholes 69 dimensioned to hold pH electrodes. Internal diameter of thehole 69 is about 1.20 cm (0.472 inch) which allows the electrode passthrough, and snugly fit within, the hole 69 of cover 63. The holes 69are located longitudinally at 0.6, 5, 10, 15, 20, and 25 cm fromlongitudinal end 92.

An absorbent structure sample having a basis weight of about 500 gramsper square inch and a density of about 0.2 g/cm³, is cut into arectangular shape with a dimension of 4.92 cm (1.94 inches) width and34.93 cm (13.75 inches) length by a textile saw, available from EastmanMachine Company in Buffalo, N.Y., under the designation Chickadee IIRotary Shear, Type D-2, 110 volt textile saw. The textile saw createssmooth edges without changing the edge density of the absorbentstructure sample. The cut piece of the absorbent structure sample isthen placed on the mesh screen forming bottom 65 of the testing chamber62.

ORION Gel-Filled Combination Electrodes model 91-35, which are availablefrom ORION Research Inc., are used for this testing. The tip of the pHelectrode 90 is covered by a cap which protects the electrode 90 andkeeps it from drying out. The cap is part of the electrodes obtainedfrom the manufacturer. The cap has to be removed before testing andsaved for storage. The tip of the electrode 90 sits on the surface ofthe absorbent structure sample in the way that the electrodes isperpendicular to the surface of the absorbent structure sample. Noadditional pressure other than the weight of the electrode 90 is neededto ensure a good contact and non-disrupted surface. The electrodes 90are separately connected to respective pH meter 91 (ORION BenchtoppH/ISE Meter, model 710A, also available from ORION Research Inc.). Themeter/electrode system is calibrated with three buffers (pH=4.01, 7.00,and 10.00, available from VWR Scientific Co. with catalogue number34170-127, 34170-130, and 34170-133 respectively) prior to the testing.

The test container 60 is then placed on inclined base 80 which isconfigured such that the bottom 74 of holding chamber 61 forms anincline angle of 30 degrees above horizontal such that the horizontalend 70 is higher than the horizontal end 71. The inclined base 80 inturn rests on laboratory jack 93. A reservoir for liquid is providedcomprising an aspirator bottle 82 including a rubber stopper 83 and anaspirator tube 84. The rubber stopper 83 has to be tightly inserted intothe aspirator bottle 82 to prevent air from leaking. The aspiratorbottle 82 is connected by supply tube 76 to holding chamber 61. Supplytube 76 is supported by clamp 85 which is attached to a laboratory stand86 in order to minimize the effect of movement of supply tube 76 onelectronic balance (scale) 81 during testing. The aspirator bottle restson an electronic balance 81. The electronic balance 81 in turn rests onlaboratory jack 87. The aspirator bottle is filled with an aqueoussolution containing 0.9 weight percent sodium chloride. The salinesolution in aspirator bottle 82 is colored with FD & C blue dye No. 1 tofacilitate/enhance measurement readings.

To start the testing procedure, testing chamber 62 and cover 63 areremoved from holding chamber 61 which remains in place on incline base80. The aspirator bottle is raised on laboratory jack 87 to an arbitraryheight. The inclined base 80 is raised on laboratory jack 93 until thesaline solution contained in aspirator bottle 82 fills the lower end(about 0.64 cm (about 0.25 inch) or holding chamber 61 to a 0.635 cm(0.25 inch) depth at its deepest point. At this point, the testingchamber 62 is placed in the holding chamber 61 but is held out ofcontact with the saline solution present in holding chamber 61 by screw78. Specifically, screw 78 is passed through threaded opening 77 untilit contacts the side of test chamber 62. The force exerted by screw 78presses test chamber 62 against holding chamber 61 and prevents the testchamber 62 from completely entering holding chamber 61. Cover 63 is thenplaced on holding chamber 61. Six pH probes 90 are inserted through theholes 69 of cover 63 until they contact the surface of the composite.The pH meters 91 are set in the measurement mode. Balance 81 is thenzeroed, and the bottom end of screen 68 is lowered into the salinesolution by releasing the force exerted by screw 78. The junction ofscreen 68 and bottom 65, and the absorbent structure sample locatedgenerally thereat, contact the saline solution. The saline solution isfed at a constant hydrostatic head from the aspirator bottle 82 into thelower end of holding chamber 61. The progress of the saline solution incentimeters, the increase in weight (as registered by balance 81), andthe readings on pH meters 91 (only those electrodes contacting saturatedabsorbent structure sample display pH values, otherwise no values areobtained), as a function of time, are recorded for a period of six hourswith, generally, periodic measurements being recorded as, for example,at two minute intervals for the first 5 readings and then ten minuteintervals for the rest of the readings. The Wicking Capacity value isdefined and normalized as the increase in liquid weight registered bybalance 81 at the end of the testing divided by the dry weight of theabsorbent structure. The pH range is presented as the minimum andmaximum pH values obtained during the testing period by any pH meters91.

Ionization Rate

The Ionization Rate test method measures the initial Ionization Rate ofan ionizable material in 0.9 weight percent sodium chloride aqueoussolution.

A stock 0.9 weight percent sodium chloride aqueous solution is preparedby dissolving 67.5 grams of sodium chloride, available from AldrichChemical Company in Milwaukee, Wis. with catalog number 22,351-4, underthe Chemical Abstract Service Registry number [7647-14-5], and with achemical purity greater than 99 percent, in 7.5 liters of ultrapurewater contained in a 18.7 liter plastic utility bucket. The ultrapurewater is obtained by filtering distilled water through a filteringsystem, available from Millipore Corporation of Bedford, Mass., underthe designation Milli-Q Reagent Water System. A Nuova II stir plate,available from Thermolyne Corporation of Dubuque, Iowa, and a 7 cm longmagnetic stir bar is used to thoroughly mix and dissolve the sodiumchloride in the ultrapure water. The 7.5 liter of stock 0.9 weightpercentage aqueous solution of sodium chloride is stirred for about 72hours. A cover is placed on top of the 18.7 liter plastic utility bucketduring the 72 hour stirring time, so as to limit dust or otherparticulate contamination of the solution, leaving only a very smallopening for air exchange. This will allow the stock saline solution toequilibrate with the carbon dioxide in the air, thus stabilizing the pHlevel of the stock saline solution.

An ORION Ross Glass Combination pH electrode model 8202BN, which isavailable from ORION Research Inc. in Boston, Mass., is used in thistesting. Referring to FIG. 3, the tip of the pH electrode 20 is coveredby a cap which protects the electrode 20 and keeps it from drying out.The cap is part of the electrode obtained from the manufacturer. Theelectrode 20 is connected to the pH meter 23 (ORION Benchtop pH/ISEMeter, model 710A, also available from ORION Research Inc.). The pHmeter 23 is interfaced to a computer 25 (such as a Compaq Portable 386available from Compaq Computer) for acquiring pH values versus time. Themeter/electrode system is calibrated with three buffers (pH=4.01, 7.00,and 10.00, available from VWR Scientific Co. with catalog numbers34170-127, 34170-130, and 34170-133 respectively) prior to the testing.The pH electrode 20 is suspended vertically in a solution being measuredby the pH electrode holder 24. Both the pH electrode reference junction21 and the pH sensing bulb 22 must be totally immersed in the solutionbeing measured in order to function properly.

The computer 25 is turned on and the data acquisition software isstarted. The sample identification is entered into the program and theprogram is set to record data every 5 seconds for a suitable length oftime.

To start the testing procedure, 200 grams of the stock 0.9 weightpercent sodium chloride aqueous solution is measured into a 250 ml glassbeaker 28 using an electronic balance (scale) available from SartoriusCorporation in Bohemia, N.Y. A mass of about 2 grams of the testingmaterial is weighed using the same electronic balance. The glass beaker28 containing the 200 grams of stock saline solution is placed on theNuova II stir plate 26 and the magnetic stir bar 27, about 3.18 cm (1.25inch) long, is placed into the beaker. The Nuova II stir plate 26 isturned on and set to a stir rate setting of 8. The pH electrode 20 isimmersed into the solution and suspended in the center of the beaker.The tip of the pH sensing bulb 22 is immersed about 3 cm (1.18 inch)into the solution. The pH meter is set in the measurement mode. When thepH value of the stock saline solution has not changed within 5 minutes,the ionization rate measurement is ready to begin.

The measurement of the ionization rate begins with the data acquisitionsoftware ready to acquire data and the stock saline solution beingcontinuously stirred at a stir rate setting of 8. Then two grams of thetesting material is poured into the beaker 28 of stock saline solution.The data acquisition software records the pH values of the solutionevery 5 seconds. The test is conducted for at least 10 minutes. A longertest time is needed for materials with weaker ionic strength and/orlower solubility in saline. The test can be stopped when the final pHvalue has stabilized for at least 2 minutes. The pH electrode 20 iscarefully removed from the solution and it is cleaned with distilledwater. The glass beaker 28 is washed with distilled water and driedcleanly. The test procedure is repeated for at least three replicatesper testing material. The Ionization Rate for each testing material willbe an average of at least three replicates.

The Ionization Rate of a material is defined as the change of the pHvalue as measured at 5 seconds relative to the full range of pH uponfull ionization as a function of time.

The Ionization Rate is calculated using the following equation:

${{Ionization}\mspace{14mu}{Rate}} = {{\frac{\frac{{p\; H_{5}} - {p\; H_{0}}}{{p\; H_{m}} - {p\; H_{0}}}}{5\mspace{14mu}\sec} \cdot \frac{60\mspace{14mu}\sec}{\min}} = {{12 \cdot {\frac{{p\; H_{5}} - {p\; H_{0}}}{{p\; H_{m}} - {p\; H_{0}}}}}\min^{- 1}}}$

-   where pH₀=pH value of the stock saline solution before the testing    material is added    -    (0 sec)    -   pH₅=pH value at 5 seconds    -   pH_(m)=For acidic materials, pH_(m) represents the minimum pH        value recorded during the evaluation. For basic materials,        pH_(m) represents the maximum pH value recorded during the        evaluation.

EXAMPLES

For use in the following examples, the following component materialswere obtained or prepared.

a. Commercial Polyacrylate Superabsorbent (Component 1)

As a control material, a commercial sodium polyacrylate superabsorbent,designated as FAVOR® 880 superabsorbent polymer, was obtained fromStockhausen, Inc. of Greensboro, N.C. This superabsorbent has a degreeof neutralization of about 70 mole percent. The superabsorbent wassieved and the 300 to 600 micrometers particle size range was used forfurther evaluation. The FAVOR® 880 superabsorbent polymer had a FreeSwell value of about 40 grams per gram and an Absorbency Under Loadvalue of about 30 grams per gram.

b. Polyacrylic Acid Gel (Component 2)

Into a 10 gallon jacketed reactor, equipped with an agitator andcontaining 24 kg of distilled water, 6 kg of acrylic acid, 10 grams ofpotassium persulfate (K₂S₂O₈), and 24 grams ofN,N′-methylenebisacrylamide, all available from Aldrich ChemicalCompany, were added and mixed at room temperature to form a completelydissolved solution. The reactor was then heated to 60° C. for at leastfour hours. The agitator was on continuously. The polyacrylic acid gelformed was cut into less than 1 inch cubes and dried in a ventilatedoven at 60° C. for at least two days. The completely dried polyacrylicacid polymer was ground into particulate by a commercial grinder (Model:C.W. Brabender Granu-Grinder) and sieved using a Sweco Separator (24inch Model), with four different particle size ranges (150 to 300micrometers, 300 to 600 micrometers, 600 to 850 micrometers, and 850 to1190 micrometers) being used for further evaluation, designated asComponents 2a, 2b, 2c, and 2d respectively. The polyacrylic acidpolymers had a Free Swell value of about 9 grams per gram and anAbsorbency. Under Load value of about 6 grams per gram.

c. Basic Second Material or Buffering Agent (Component 3)

Granular sodium hydrogen carbonate (NaHCO₃), available from AldrichChemical Company, was obtained and sieved, with the 300 to 600micrometers particle size range being used for further evaluation.

d. Basic Second Material (Component 4)

Granular sodium carbonate (Na₂CO₃), available from Aldrich ChemicalCompany, was obtained and sieved, with the 300 to 600 micrometersparticle size range being used for further evaluation.

e. Acidic Second Material or Buffering Agent (Component 5)

Granular anhydrous citric acid (HOOCCH₂C(OH)(COOH)CH₂COOH), availablefrom Archer Daniels Midland Company, was obtained and sieved, with the300 to 600 micrometers, 600 to 850 micrometers, and 850 to 1190micrometers particle size ranges being used for further evaluation,designated as Components 5a, 5b, and 5c respectively.

f. Wood Pulp Fluff (Component 6)

Commercial kraft wood pulp fluff, consisting of about 16 weight percentsouthern hardwood and about 84 weight percent southern softwood, wasobtained from Alliance Paper Company, Coosa Pines, Ala., under thedesignation CR 1654 wood pulp fluff, and used as a fibrous matrixcontainment material to make absorbent structures for furtherevaluation. The wood pulp fluff had a Free Swell value of about 6 gramsper gram and an Absorbency Under Load value of about 4 grams per gram.

g. Wood Pulp Fluff (Component 7)

Commercial wood pulp fluff, consisting of about 10 weight percenthardwood and about 90 weight percent southern softwood, was obtainedfrom Weyerhaeuser Company, Mississippi, under the designation NB 416wood pulp fluff, and used as a fibrous matrix containment material tomake absorbent structures for further evaluation. The wood pulp fluffhad a Free Swell value of about 6 grams per gram and an Absorbency UnderLoad value of about 4 grams per gram.

h. PolyDiallyl Dimethyl Ammonium Hydroxide (Component 8)

About 2.1 grams of methylenebisacrylamide was dissolved as thecrosslinking agent in 370 milliliters of 60 percent by weight aqueoussolution of diallyldimethylammonium chloride monomer in a 1000milliliters conical flask. The solution was purged with nitrogen for 15minutes and the conical flask was stoppered and placed in a water bathat 60° C. Polymerization was initiated by the addition of 0.4 gram ofpotassium persulfate and 1.5 grams of sodium bisulfite to the reactionmixture. Polymerization was continued for 12 hours at 60° C. followed bycutting the gel that was formed into small pieces (about one inchcubes). The gel pieces were washed with 2 percent by weight sodiumhydroxide solution until all the chloride ions in the polymer wereexchanged for hydroxide ions. The completion of exchange was confirmedby testing the effluent after treatment with acidified silver nitrate todetect for chloride ions. The absence of chloride ions was taken as anindication of completion of conversion to the desired hydroxide form.The gel was washed thoroughly with distilled water until the pH of thedistilled water after washing was the same as the water used forwashing. The gel was dried at 50° C. overnight and ground using ablender from Warring (Model 34BL97). The ground polymer was sieved andthe four different particle size ranges (300 to 600 micrometers, 600 to850 micrometers, and 850 to 1190 micrometers) were used for furtherevaluation, designated as Components 8a, 8b, and 8c respectively. Thepolydiallyl dimethyl ammonium hydroxide polymer had a Free Swell valueof about 26 grams per gram and an Absorbency Under Load value of about18 grams per gram.

i. Chitosan (Component 9)

Forty grams of chitosan flake, available from Vanson Company under thedesignation VSN-608 chitosan, were mixed with 2000 grams of 1 weightpercent acetic acid solution in a mixer manufactured by KitchenAid(Model K45SS). About 0.3 gram of poly(ethylene glycol) diglycidyl etherhaving a molecular weight of about 400 was added into the chitosanacetate solution as a crosslinking agent. The solution was then dried at60° C. for at least 30 hours and ground into particulate and thensieved, with the 300 to 600 micrometers particle size range being usedfor further evaluation. The chitosan acetate particles were suspended ina 1 weight percent sodium hydroxide solution with a ratio of a gram ofchitosan acetate to 100 grams of the sodium hydroxide solution. Undercontinuous stirring, using a magnetic stirrer, the chitosan acetate wasconverted into chitosan within at least 5 hours. The treated chitosanparticles were then washed with distilled water four times with a ratioof chitosan to water 1 to 1000 to completely remove residual sodiumacetate and sodium hydroxide. The washed chitosan was dried at 80° C.The chitosan polymer had a Free Swell value of about 3 grams per gramand an Absorbency Under Load value of about 2 grams per gram.

Example 1

Absorbent structures were prepared using an air-laying process. Theabsorbent structures had a basis weight of about 500 grams per squaremeter, a density of about 0.2±0.01 gram per cubic centimeter, andgenerally comprised about 37 weight percent of particulate material(Components 1-5,8-9) and about 63 weight percent of wood pulp fluff(Components 6, 7). The composition of each absorbent structure sample issummarized in Table 1 and Table 2.

The absorbent structures were densified by a laboratory press, availablefrom Fred S. Carver, Inc. in Wabash, Ind., under the designation Model2333 laboratory press, at room temperature under about 10,000 to 15,000psi for about 10 seconds. The absorbent structures were cut into 2inches by 13.75 inches samples using a textile saw, available fromEastman Machine Company in Buffalo, N.Y., under the designationChickadee II Rotary Shear, Type D-2, 110 volt textile saw.

The density of each absorbent structure sample was measured by itsthickness before the absorbency and pH evaluations. If the density istoo low, the absorbent structure sample was re-densified to theacceptable range. The absorbent structure sample was then placed intothe testing apparatus for measurement of pH profile and Wicking Capacityvalues. In this example, the pH profile and the Wicking Capacity valueswere measured using a time period of 2.5 hours, instead of the 6 hourtime period as specified in the Test Methods section. The results of theevaluations are shown in Table 3. Several of the absorbent structuresamples were also evaluated for absorbency and pH properties over a timeperiod of about 24 hours. For absorbent structure samples that did notcomprise a buffering agent, or an effective amount of a buffering agent,wider variations in the pH values were generally observed over the 24hour time period as compared to the 2.5 hour time period. For absorbentstructure samples of the present invention that did comprise aneffective amount of a buffering agent, no substantial difference in theliquid absorbency or pH values was observed between the two timeperiods.

TABLE 1 Absorbent Structure Sample No. Component 1 Component 2bComponent 3 Component 4 Component 5a Component 6 *Sample 1 18.5 g — — —— 31.2 g *Sample 2 — 18.5 g — — — 31.2 g *Sample 3 — — 18.5 g  — — 31.2g *Sample 4 —  9.6 g 8.9 g — — 31.2 g Sample 5 —  9.6 g 8.9 g —  1.0 g31.2 g *Sample 6 — 10.2 g 8.3 g — — 31.2 g *Sample 7 — 10.2 g 8.3 g — 0.5 g 31.2 g Sample 8 — 10.2 g 8.3 g —  1.8 g 31.2 g Sample 9 — 10.2 g8.3 g —  3.5 g 31.2 g Sample 10 — 10.2 g 8.3 g —  5.0 g 31.2 g *Sample11 — 10.2 g 8.3 g — 10.0 g 31.2 g *Sample 12 — 10.7 g — 7.8 g — 31.2 g*Sample 13 — 10.7 g — 7.8 g  1.8 g 31.2 g Sample 14 — 10.7 g — 7.8 g10.0 g 31.2 g *Not an example of the present invention.

TABLE 2 Absorbent Structure Sample No. Comp. 1 Comp. 8 Comp. 9 Comp. 2bComp. 5a Comp. 3 Comp. 6 Comp. 7 *Sample 15 — 18.5 g — — — — — 31.2 g*Sample 16 — — 18.5 g — — — 31.2 g — *Sample 17 — — — 18.5 g  — — 31.2 g— *Sample 18 — 12.3 g — 6.3 g — — — 31.2 g Sample 19 — 12.3 g — 6.3 g0.5 g — — 31.2 g Sample 20 — 12.3 g — 6.3 g 2.0 g — — 31.2 g *Sample 21— 12.3 g — 6.3 g 3.0 g — — 31.2 g *Sample 22 — — 11.2 g 7.3 g — — 31.2 g— Sample 23 — — 11.2 g 7.3 g — 0.5 g 31.2 g — *Sample 24 — — 11.2 g 7.3g — 2.0 g 31.2 g — *Not an example of the present invention.

TABLE 3 Absorbent Structure Sample Wicking Capacity pH Range pH Range pHRange pH Range pH Range pH Range No. value (g/g) at 0.6 cm at 5 cm at 10cm at 15 cm at 20 cm at 25 cm *Sample 1 12.5 5.8-5.9 5.9-6.0 5.6-5.85.1-5.8 4.3-5.6 4.7-5.5 *Sample 2 6.8 2.2-2.5 2.5-2.8 2.7-2.8 2.3-2.42.0-2.1 — *Sample 3 4.8 7.6-7.8 8.2-8.3 7.7-8.6 7.1-8.5 6.8-8.3 7.7-8.9*Sample 4 10.5 5.5-6.5 4.9-6.5 5.2-6.9 5.8-6.9 7.2-8.4 7.2-8.7 Sample 59.9 4.2-5.0 4.7-6.3 4.9-6.1 5.9-6.7 6.3-6.8 6.4-7.3 *Sample 6 11.95.3-6.0 4.3-5.5 5.1-6.3 6.0-6.2 5.9-7.1 6.7-8.1 *Sample 7 12.1 4.7-5.24.8-6.0 6.1-6.4 4.7-6.8 5.5-7.8 6.5-8.4 Sample 8 10.9 5.3-6.4 5.5-6.55.9-6.6 4.3-6.4 6.3-6.8 5.2-6.0 Sample 9 11.2 4.6-5.0 4.4-5.6 5.4-6.33.3-5.6 4.2-5.6 4.0-5.7 Sample 10 9.8 4.6-5.3 4.4-6.1 4.9-5.9 5.2-5.34.6-4.9 4.4-4.7 *Sample 11 8.7 3.4-3.5 4.3-4.5 3.5-3.8 1.6-4.0 3.3-4.72.6-4.1 *Sample 12 11.0 5.0-8.0 4.5-6.4 5.2-7.2 6.4-7.5 6.9-7.5 8.1-8.8*Sample 13 9.2 4.3-5.8 4.3-6.3 4.8-6.3 5.8-6.5 7.1-7.6 7.4-8.3 Sample 149.9 4.2-4.6 4.5-5.2 4.5-4.9 4.7-4.8 5.0-5.6 4.8-5.2 *Sample 15 12.69.3-9.7 9.5-9.7 9.4-9.6 9.0-9.7 — — *Sample 16 5.8 7.1-8.6 6.8-8.77.3-8.4 7.4-8.1 7.9-8.4 — *Sample 17 6.8 2.2-2.5 2.5-2.8 2.7-2.8 2.3-2.42.0-2.1 — *Sample 18 10.5 5.6-8.6 5.0-7.3 4.3-5.9 6.7-8.6 6.1-6.2 —Sample 19 9.9 4.6-5.6 4.7-6.1 5.4-6.7 5.1-5.9 4.9-5.4 — Sample 20 7.74.3-5.1 4.1-5.1 3.4-3.7 3.4-3.5 — — *Sample 21 6.8 3.7-4.6 3.8-4.23.0-3.3 2.9-3.5 3.2-3.3 — *Sample 22 9.6 4.3-5.8 4.1-5.6 4.2-5.4 2.7-5.34.2-4.7 — Sample 23 9.7 5.4-5.7 4.6-5.3 5.1-6.1 5.6-7.2 5.7-5.8 —*Sample 24 8.6 5.9-6.5 5.6-6.1 6.2-6.7 7.1-9.2 7.6-9.5 — *Not an exampleof the present invention.

Example 2

Absorbent structures were prepared using an air-laying process. Theabsorbent structures had a basis weight of about 500 grams per squaremeter, a density of about 0.2±0.01 gram per cubic centimeter, andgenerally comprised about 37 weight percent of particulate material(Components 2, 3, 5, and 8) and about 63 weight percent of wood pulpfluff (Component 6). The composition of each absorbent structure sampleis summarized in Table 4.

The absorbent structures were densified by a laboratory press, availablefrom Fred S. Carver, Inc. in Wabash, Ind., under the designation Model2333 laboratory press, at room temperature under about 10,000 to 15,000psi for about 10 seconds. The absorbent structures were cut into 2inches by 13.75 inches samples using a textile saw, available fromEastman Machine Company in Buffalo, N.Y., under the designationChickadee II Rotary Shear, Type D-2, 110 volt textile saw.

The density of each absorbent structure sample was measured by itsthickness before the absorbency and pH evaluations. If the density istoo low, the absorbent structure sample was re-densified to theacceptable range. The absorbent structure sample was then placed intothe testing apparatus for measurement of pH profile and Wicking Capacityvalues. The results of the evaluations are shown in Table 5. In Table 5,I_(a) represents the Ionization Rate for the acidic polymer or acidicsecond material used in the absorbent structure sample. I_(b) representsthe Ionization Rate for the basic polymer or basic second material usedin the absorbent structure sample.

TABLE 4 Absorbent Structure Sample Component Component ComponentComponent Component Component No. 2a 2b 2c 2d 3 6 Sample 25 10.2 g — — —8.5 g 31.2 g *Sample 26 — 10.2 g — — 8.5 g 31.2 g *Sample 27 — — 10.2g —8.5 g 31.2 g *Sample 28 — — — 10.2 g 8.5 g 31.2 g Component ComponentComponent Component Component 8a 8b 8c 5c 6 *Sample 29  9.5 g — — — 9.0g 31.2 g Sample 30 —  9.5 g — — 9.0 g 31.2 g *Sample 31 — —  9.5 g — 9.0g 31.2 g *Not an example of the present invention.

TABLE 5 Absorbent Wicking pH pH pH pH pH pH Structure I_(a) I_(b)Capacity Range at Range at Range at Range at Range at Range at SampleNo. (min⁻¹) (min⁻¹) value (g/g) 0.6 cm 5 cm 10 cm 15 cm 20 cm 25 cmSample 25 9.01 8.94 12.0 5.6-6.3 5.4-6.4 6.2-6.9 6.9-7.0 7.1-7.5 5.9-7.7*Sample 26 8.72 8.94 11.2 5.4-6.6 5.6-6.7 5.9-6.8 6.4-6.8 6.6-7.66.3-8.5 *Sample 27 6.75 8.94 9.4 5.8-6.5 5.1-6.8 5.8-6.7 6.4-7.5 7.2-7.96.6-8.9 *Sample 28 6.64 8.94 9.7 4.7-7.1 5.1-6.7 4.8-7.1 6.8-7.5 7.8-8.87.7-8.3 *Sample 29 9.74 12.00 7.7 4.5-8.7 4.2-5.2 3.7-5.4 3.7-4.93.1-3.4 3.6-4.9 Sample 30 9.74 9.78 7.3 3.8-4.7 4.1-5.1 3.6-4.7 3.2-4.13.3-4.0 — *Sample 31 9.74 8.51 8.1 4.6-5.4 2.5-4.3 3.2-5.0 2.8-4.43.2-3.5 4.3-6.6 *Not an example of the present invention.

Example 3

Absorbent structures were prepared using an air-laying process. Theabsorbent structures had two layers and a total basis weight of about500 grams per square meter, a density of about 0.2±0.01 gram per cubiccentimeter, and generally comprised about 37 weight percent ofparticulate material (Components 2, 3, 5, and 8) and about 63 weightpercent of wood pulp fluff (Component 6). The two layered absorbentstructure was made by first forming a lower layer of particulatematerial and wood pulp fluff, and then forming an upper layer ofparticulate material and wood pulp fluff on the top of the lower layer.The composition of each absorbent structure sample is summarized inTable 6.

The layered absorbent structures were densified by a laboratory press,available from Fred S. Carver, Inc. in Wabash, Ind., under thedesignation Model 2333 laboratory press, at room temperature under about10,000 to 15,000 psi for about 10 seconds. The absorbent structures werecut into 2 inches by 13.75 inches samples using a textile saw, availablefrom Eastman Machine Company in Buffalo, N.Y., under the designationChickadee II Rotary Shear, Type D-2, 110 volt textile saw.

The density of each absorbent structure sample was measured by itsthickness before the absorbency and pH evaluations. If the density istoo low, the absorbent structure sample was re-densified to theacceptable range. The absorbent structure sample was then placed intothe testing apparatus for measurement of pH profile and Wicking Capacityvalues. The surface of the upper layer is in contact with the pH probes.The results of the evaluations are shown in Table 7.

While the present invention has been described in terms of the specificembodiments described above, numerous equivalent changes andmodifications will be clear to those skilled in the art. Accordingly,the specific examples set forth above are not intended to limit in anymanner the scope of the invention as set forth in the appended claims.

TABLE 6 Absorbent Component 2b Component 3 Component 8c Component 5cComponent 6 Structure Sample Upper Lower Upper Lower Upper Lower UpperLower Upper Lower No. Layer Layer Layer Layer Layer Layer Layer LayerLayer Layer *Sample 32 10.2 — — 8.5 — — — — 15.6 15.6 *Sample 33 — 10.28.5 — — — — — 15.6 15.6 Sample 34 10.2 — — 8.5 — — — — 20.8 10.4 *Sample35 — 10.2 8.5 — — — — — 10.4 20.8 *Sample 36 — — — — 9.5 — — 9.0 15.615.6 *Sample 37 — — — — — 9.5 9.0 — 15.6 15.6 Sample 38 — — — — 9.5 — —9.0 20.8 10.4 *Sample 39 — — — — — 9.5 9.0 — 10.4 20.8 *Not an exampleof the present invention.

TABLE 7 Absorbent Structure Sample Wicking Capacity pH Range pH Range pHRange pH Range pH Range pH Range No. value (g/g) at 0.6 cm at 5 cm at 10cm at 15 cm at 20 cm at 25 cm *Sample 32 9.0 3.6-4.4 3.9-6.6 4.1-7.36.1-7.3 7.0-8.4 5.3-7.9 *Sample 33 9.4 6.0-7.5 4.2-7.0 5.6-7.8 6.2-7.27.0-8.2 7.1-8.4 Sample 34 10.0 3.4-4.4 3.9-5.3 4.3-6.1 6.2-7.0 7.0-7.76.5-7.8 *Sample 35 8.1 5.8-7.5 5.7-7.0 5.5-6.9 6.1-7.3 4.8-7.4 5.7-8.1*Sample 36 7.8 5.0-6.3 3.9-5.5 4.0-6.3 2.9-4.3 2.9-3.6 3.8-4.7 *Sample37 7.7 2.1-3.3 2.2-3.7 2.3-3.9 2.4-3.7 3.0-3.2 3.0-3.3 Sample 38 9.34.3-5.8 3.5-5.4 3.4-5.9 3.4-4.8 3.8-4.6 3.6-3.8 *Sample 39 7.9 2.2-4.82.8-6.3 2.7-6.4 2.7-4.7 3.0-3.7 — *Not an example of the presentinvention.

1. An absorbent structure having an upper surface, the absorbentstructure comprising: a) a water-swellable, water-insoluble polymerhaving acidic functional groups, wherein the water-swellable,water-insoluble polymer has at least about 50 molar percent of theacidic functional groups in free acid form; and b) a polymeric basicmaterial that is not water-swellable and water-insoluble; wherein theabsorbent structure exhibits a Wicking Capacity value that is at leastabout 5 grams per gram of absorbent structure and exhibits a pH on theupper surface that remains within the range of about 3 to about
 8. 2.The absorbent structure of claim 1 wherein the polymeric basic materialcomprises polyamine, polyimine, polyamide, chitin, chitosan,polyquaternary ammonium, polyasparagine, polyglutamine, polylysine,polyarginine, and mixtures thereof.
 3. The absorbent structure of claim1 wherein the acidic water-swellable, water-insoluble polymer has a pKabetween about 2 and about
 10. 4. The absorbent structure of claim 1wherein the acidic water-swellable, water-insoluble polymer has at leastabout 70 molar percent of the acidic functional groups in free acidform.
 5. The absorbent structure of claim 1 wherein the acidicwater-swellable, water-insoluble polymer is prepared from a basecomprising polyacrylamide, polyvinyl alcohol, ethylene maleic anhydridecopolymer, polyvinylether, polyacrylic acid, polyvinylpyrrolidone,polyvinylmorpholine, carboxymethyl cellulose, carboxymethyl starch,hydroxypropyl cellulose, algin, alginate, carrageenan, acrylic graftedstarch, acrylic grafted cellulose, polyaspartic acid, polyglutamic acid,and copolymers thereof.
 6. The absorbent structure of claim 1 whereinthe acidic water-swellable, water-insoluble polymer comprises carboxylgroups, sulfonic groups, sulphate groups, sulfite groups, phosphategroups, and combinations thereof.
 7. The absorbent structure of claim 5wherein the acidic water-swellable, water-insoluble polymer and thebasic material are present in the absorbent structure in a molar ratiofrom about 10:1 to about 1:10.
 8. The absorbent structure of claim 5wherein the absorbent structure exhibits a Wicking Capacity value thatis at least about 10 grams per gram of absorbent structure.
 9. Theabsorbent structure of claim 5 wherein the absorbent structure exhibitsa pH on the upper surface that remains within the range of about 4 toabout
 7. 10. The absorbent structure of claim 1 further comprising aliquid-permeable topsheet and a backsheet attached to the topsheet,wherein the absorbent structure is positioned between theliquid-permeable topsheet and the backsheet.